I am hoping this is what you are looking for, I have never used GDB before:

#0  0x00007ffff79de727 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
No symbol table info available.
#1  0x00007ffff78ced15 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
No symbol table info available.
#2  0x00007ffff78e1f9a in ?? () from /lib/x86_64-linux-gnu/libc.so.6
No symbol table info available.
#3  0x00005555555811fb in cprintf (
    format=0x5555556c8f6f "%d Cannot open '%s': %s\n") at server/sysdep.c:436
        arg_ptr = {{gp_offset = 32, fp_offset = 48,
            overflow_arg_area = 0x7ffff474e900,
            reg_save_area = 0x7ffff474e840}}
        buf = "571 Cannot open 'messages/hello': tôÿ200\000îÿ/ ó%7`ætôÿ\177\000\000\000îÿ/ ó%7ÿ\003\000\000\000\000\000\000žD\226÷ÿ\177\000\000h\205lUUU\000\000Hætôÿ\177\000\000\020\000\000\000ÿ\177\000\000ðätôÿ\177\000\000°ätôÿ\177\000\000\000îÿ/ ó%7\001\200­û!\000\000\000`ætôÿ\177\000\000@\023Á÷ÿ\177\000\000`ætôÿ\177\000\000`ætôÿ\177\000\000\212ætôÿ\177\000\000_êtôÿ\177\000\000¿bÀ÷ÿ\177\000\000\020\000\000\000ÿ\177\000\000"...
        rc = 0
#4  0x000055555559c322 in cmd_mesg (mname=0x7ffff474eb95 "hello")
    at server/modules/ctdlproto/serv_file.c:551
        mfp = 0x0
        targ = "messages/hello\000\000Pétôÿ\177\000\000\000îÿ/ ó%7`êtô\000\000\000\000\000\001\000\000\000\000\000\000à{\002ìÿ\177\000\000ðÎ\000ìÿ\177\000\000\000\000\000\000\000\000\000\000`±\001ìÿ\177\000\000\220\n\001ìÿ\177\000\000¿bÀ÷ÿ\177\000\000\020\000\000\000ÿ\177\000\000ðétôÿ\177\000\000°étôÿ\177\000\000\000îÿ/_\000\000\000P[\000ìÿ\177\000\000î_Â÷ÿ\177", '\000' <repeats 18 times>, "Û\036\003ìÿ\177", '\000' <repeats 18 times>, "z¹Â÷ÿ\177\000\000ÐH\003ìÿ\177\000\000\000\000\000\000\000\000\000\000"...
        buf = "hello\000\000\000\220²\001ìÿ\177\000\000\000\000\000\000\000\000\000\000\202FÂ÷ÿ\177\000\000\000\000\000\000\000\000\000\000âEÂ÷ÿ\177\000\000ÙhlUUU\000\000\000îÿ/ ó%7", '\000' <repeats 16 times>, "°ûtôÿ\177\000\000>Óÿÿÿ\177\000\000?Óÿÿÿ\177\000\000àÓÿÿÿ\177\000\000\200ýtôÿ\177\000\000\212â\226÷ÿ\177\000\000\020\000\000\000\060\000\000\000\220ëtôÿ\177\000\000Ðêtôÿ\177\000\000\000îÿ/ ó%7Pëtôÿ\177\000\000(|\002ìÿ\177\000\000L|\002ìÿ\177\000\000\000\000\000\000\000\000\000\000\220ëtôÿ\177\000\000"...
        dp = 0x7fffec001010
        d = 0x555555771ee0
#5  0x000055555558c4ad in DLoader_Exec_Cmd (cmdbuf=0x7ffff474eb90 "MESG hello") at server/serv_extensions.c:219
        vP = 0x55555577eaf0
        p = 0x55555577eaf0
#6  0x0000555555592582 in do_command_loop () at server/modules/ctdlproto/serv_ctdlproto.c:64
        cmdbuf = "MESG hello", '\000' <repeats 4085 times>
#7  0x00005555555824e1 in worker_thread (blah=0x0) at server/sysdep.c:937
        sockets_waiting = 1
        highest = 37
        ptr = 0x0
        worker_session = 0x7fffec027b80
        readfds = {__fds_bits = {137438953472, 0 <repeats 15 times>}}
        tv = {tv_sec = 0, tv_usec = 999995}
        force_purge = 0
        serviceptr = 0x0
        ssock = 37
        con = 0x7fffec027b80
        i = 1
#8  0x00007ffff7bfb609 in start_thread () from /lib/x86_64-linux-gnu/libpthread.so.0
No symbol table info available.
#9  0x00007ffff7975133 in clone () from /lib/x86_64-linux-gnu/libc.so.6
No symbol table info available.

I ran the GDB instructions again and I believe I have a better backtrace than the one I posted.  Any ideas?

Thread 3 (Thread 0x7ffff3f4d700 (LWP 18942)):
#0  0x00007ffff79de947 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
No symbol table info available.
#1  0x00007ffff78ced15 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
No symbol table info available.
#2  0x00007ffff78e1f9a in ?? () from /lib/x86_64-linux-gnu/libc.so.6
No symbol table info available.
#3  0x00005555555811fb in cprintf (format=0x5555556c8f6f "%d Cannot open '%s': %s\n") at server/sysdep.c:436
        arg_ptr = {{gp_offset = 32, fp_offset = 48, overflow_arg_area = 0x7ffff3f4b900, reg_save_area = 0x7ffff3f4b840}}
        buf = "571 Cannot open 'messages/hello': ôóÿ200\000\236Ýi­pÃ%`¶ôóÿ\177\000\000\000\236Ýi­pÃ%ÿ\003\000\000\000\000\000\000ØF\226÷ÿ\177\000\000h\205lUUU\000\000H¶ôóÿ\177\000\000\020\000\000\000ÿ\177\000\000ðŽôóÿ\177\000\000°Žôóÿ\177\000\000\000\236Ýi­pÃ%\001\200­û!\000\000\000`¶ôóÿ\177\000\000@\023Á÷ÿ\177\000\000`¶ôóÿ\177\000\000`¶ôóÿ\177\000\000\212¶ôóÿ\177\000\000_ºôóÿ\177\000\000¿bÀ÷ÿ\177\000\000\020\000\000\000ÿ\177\000\000"...
        rc = 0
#4  0x000055555559c322 in cmd_mesg (mname=0x7ffff3f4bb95 "hello") at server/modules/ctdlproto/serv_file.c:551
        mfp = 0x0
        targ = "messages/hello\000\000P¹ôóÿ\177\000\000\000\236Ýi­pÃ%\000\000\000\000\000\000\000\000\000\001\000\000\000\000\000\000\220Ò\000äÿ\177\000\000àŒ\000äÿ\177\000\000\000\000\000\000\000\000\000\000Ðq\001ìÿ\177\000\000ÀÏ\000äÿ\177\000\000¿bÀ÷ÿ\177\000\000\020\000\000\000ÿ\177\000\000ð¹ôóÿ\177\000\000°¹ôóÿ\177\000\000\000\236Ýi_\000\000\000\060\032\000äÿ\177\000\000î_Â÷ÿ\177", '\000' <repeats 18 times>, "k\027\000äÿ\177", '\000' <repeats 18 times>, "z¹Â÷ÿ\177", '\000' <repeats 18 times>...
        buf = "hello\000\000\000\060\060\000äÿ\177\000\000\000\000\000\000\000\000\000\000ìFÂ÷ÿ\177\000\000\000\000\000\000\000\000\000\000âEÂ÷ÿ\177\000\000ÙhlUUU\000\000\000\236Ýi­pÃ%", '\000' <repeats 16 times>, "°Ëôóÿ\177\000\000ÎÓÿÿÿ\177\000\000ÏÓÿÿÿ\177\000\000pÔÿÿÿ\177\000\000\200Íôóÿ\177\000\000ªä\226÷ÿ\177\000\000\020\000\000\000\060\000\000\000\220»ôóÿ\177\000\000кôóÿ\177\000\000\000\236Ýi­pÃ%P»ôóÿ\177\000\000ØÒ\000äÿ\177\000\000üÒ\000äÿ\177\000\000\000\000\000\000\000\000\000\000\220»ôóÿ\177\000\000"...
        dp = 0x7
        d = 0x7fffe40008d0
#5  0x000055555558c4ad in DLoader_Exec_Cmd (cmdbuf=0x7ffff3f4bb90 "MESG hello") at server/serv_extensions.c:219
        vP = 0x555555782ee0
        p = 0x555555782ee0
#6  0x0000555555592582 in do_command_loop () at server/modules/ctdlproto/serv_ctdlproto.c:64
        cmdbuf = "MESG hello", '\000' <repeats 4085 times>
#7  0x00005555555824e1 in worker_thread (blah=0x0) at server/sysdep.c:937
        sockets_waiting = 1
        highest = 37
        ptr = 0x0
        worker_session = 0x7fffe400d230
        readfds = {__fds_bits = {137438953472, 0 <repeats 15 times>}}
        tv = {tv_sec = 0, tv_usec = 999997}
        force_purge = 0
        serviceptr = 0x0
        ssock = 37
        con = 0x7fffe400d230
        i = 1
#8  0x00007ffff7bfb609 in start_thread () from /lib/x86_64-linux-gnu/libpthread.so.0
No symbol table info available.
#9  0x00007ffff7975353 in clone () from /lib/x86_64-linux-gnu/libc.so.6
No symbol table info available.

Thread 2 (Thread 0x7ffff474e700 (LWP 18941)):
#0  0x00007ffff796b1eb in select () from /lib/x86_64-linux-gnu/libc.so.6
No symbol table info available.
#1  0x0000555555581fe4 in worker_thread (blah=0x0) at server/sysdep.c:817
        sockets_waiting = 0
        highest = 36
        ptr = 0x0
        worker_session = 0x0
        readfds = {__fds_bits = {137422176256, 0 <repeats 15 times>}}
        tv = {tv_sec = 0, tv_usec = 998444}
        force_purge = 0
        serviceptr = 0x0
        ssock = 37
        con = 0x7fffec037090
        i = 1
#2  0x00007ffff7bfb609 in start_thread () from /lib/x86_64-linux-gnu/libpthread.so.0
No symbol table info available.
#3  0x00007ffff7975353 in clone () from /lib/x86_64-linux-gnu/libc.so.6
No symbol table info available.

Thread 1 (Thread 0x7ffff4915980 (LWP 18936)):
#0  0x00007ffff793323f in clock_nanosleep () from /lib/x86_64-linux-gnu/libc.so.6
No symbol table info available.
#1  0x00007ffff7938ec7 in nanosleep () from /lib/x86_64-linux-gnu/libc.so.6
No symbol table info available.
#2  0x00007ffff796ba7f in usleep () from /lib/x86_64-linux-gnu/libc.so.6
No symbol table info available.
#3  0x0000555555582afc in go_threading () at server/threads.c:142
No locals.
#4  0x0000555555572c24 in main (argc=1, argv=0x7fffffffe678) at server/server_main.c:308
        facility = " M\000\000\000\000\000\000 ]\000\000\000\000\000\000 ]\000\000\000\000\000\000à\002\000\000\000\000\000"
        pw = {pw_name = 0x7fffffffd560 "citadel", pw_passwd = 0x7fffffffd568 "x", pw_uid = 1001, pw_gid = 1002, pw_gecos = 0x7fffffffd574 "Citadel service account", pw_dir = 0x7fffffffd58c "/usr/local/citadel", pw_shell = 0x7fffffffd59f "/usr/local/citadel/citadel"}
        pwp = 0x7fffffffd510
        pwbuf = "citadel\000x\000\061\060\060\061:1002:Citadel service account\000/usr/local/citadel\000/usr/local/citadel/citadel\000\000\000\026\000\000\000\b\000\000\000\t\000\000\000\000\000\000\200\000\020@\002\001\000\202\022\004\004@\000\200\000@\b\000\000@\000\b\000\000\b8@\002 Z\210\201\a\004\002\220\022\bá¿\030\000à\002YÐÞÿÿÿ\177\000\000@ãÿÿÿ\177", '\000' <repeats 11 times>, "\060\222ôÿ\177\000\000\212Ï\221ôÿ\177\000\000\000\000\000\000\000\000\000\000"...
        drop_root_perms = 1
        max_log_level = 6
        ctdldir = 0x5555556c1b41 "/usr/local/citadel"
        syslog_facility = 24
        u = 1001
        p = 0x7ffff7a45460
        g = -1

We might be able to work with that.  At least we have line numbers and calls to look at.  That's an odd place for it to have trouble though.    What operating system are you on?

It's a weird one because nothing unusual is happening here, it's just trying to display the site's greeting banner (hello).

What is the contents of ./messages/hello ?   Is it there?  Is it corrupted?

Ok, I'm really struggling with this one because it's crashing while doing something very clean and simple, which may suggest a problem elsewhere.

I've added a few more diagnostics to that area of the system.  Also it will now print a simple stack trace to the syslog at the moment it crashes.   Please upgrade to version 1013 and see if the behavior changes any.

Also if you find you need to run it in the debugger, the command to produce the output we need is "thread apply all bt" after it crashes.

I am running on Debian Bullseye/Sid. 

As far as the messages folder... It's empty, I'm guessing that's not right.

I reloaded with Easyinstall and apparently it's still listing the server version as 1011.  Very odd though, the server stayed up for hours without a problem until I opened Webcit in my browser, which came up to my personal mail room and then errored saying it couldn't stay connected to the Citadel server.  Of course checking the logs immediately showed a citserver crash.  I've listed the stack trace from the logs and the surrounding logs below:

May 23 20:25:08 nadia webcit[615]: webcit[615]: SSL_accept (sock=5) success, usi
ng TLS_AES_128_GCM_SHA256 on TLSv1.3 (128 of 128 bits)
May 23 20:25:08 nadia webcit[615]: SSL_accept (sock=5) success, using TLS_AES_12
8_GCM_SHA256 on TLSv1.3 (128 of 128 bits)
May 23 20:25:08 nadia webcit[615]: webcit[615]: Ending SSL/TLS
May 23 20:25:08 nadia webcit[615]: Ending SSL/TLS
May 23 20:25:08 nadia webcit[615]: webcit[615]: SSL_accept (sock=13) success, us
ing TLS_AES_128_GCM_SHA256 on TLSv1.3 (128 of 128 bits)
May 23 20:25:08 nadia webcit[615]: webcit[615]: SSL_accept (sock=8) success, usi
ng TLS_AES_128_GCM_SHA256 on TLSv1.3 (128 of 128 bits)
May 23 20:25:08 nadia webcit[615]: SSL_accept (sock=13) success, using TLS_AES_1
28_GCM_SHA256 on TLSv1.3 (128 of 128 bits)
May 23 20:25:08 nadia webcit[615]: SSL_accept (sock=8) success, using TLS_AES_12
8_GCM_SHA256 on TLSv1.3 (128 of 128 bits)
May 23 20:25:08 nadia webcit[615]: webcit[615]: SSL_accept (sock=6) success, usi
ng TLS_AES_128_GCM_SHA256 on TLSv1.3 (128 of 128 bits)

May 23 20:25:08 nadia webcit[615]: webcit[615]: SSL_accept (sock=6) success, usi
ng TLS_AES_128_GCM_SHA256 on TLSv1.3 (128 of 128 bits)
May 23 20:25:08 nadia webcit[615]: SSL_accept (sock=6) success, using TLS_AES_12
8_GCM_SHA256 on TLSv1.3 (128 of 128 bits)
May 23 20:25:08 nadia citserver[9135]: citserver[9135]: SIGSEGV received in thre
ad 140169230857984 at address 0xb79be820
May 23 20:25:08 nadia citserver[9135]: SIGSEGV received in thread 14016923085798
4 at address 0xb79be820
May 23 20:25:08 nadia webcit[615]: webcit[615]: Ending SSL/TLS
May 23 20:25:08 nadia webcit[615]: Ending SSL/TLS
May 23 20:25:08 nadia webcit[615]: webcit[615]: Ending SSL/TLS
May 23 20:25:08 nadia webcit[615]: Ending SSL/TLS
May 23 20:25:08 nadia webcit[615]: webcit[615]: SSL_accept (sock=9) success, usi
ng TLS_AES_128_GCM_SHA256 on TLSv1.3 (128 of 128 bits)
May 23 20:25:08 nadia webcit[615]: webcit[615]: SSL_accept (sock=10) success, us
ing TLS_AES_128_GCM_SHA256 on TLSv1.3 (128 of 128 bits)
May 23 20:25:08 nadia webcit[615]: SSL_accept (sock=10) success, using TLS_AES_1
28_GCM_SHA256 on TLSv1.3 (128 of 128 bits)
May 23 20:25:08 nadia webcit[615]: SSL_accept (sock=9) success, using TLS_AES_12
8_GCM_SHA256 on TLSv1.3 (128 of 128 bits)
May 23 20:25:08 nadia webcit[615]: webcit[615]: Ending SSL/TLS
May 23 20:25:08 nadia webcit[615]: webcit[615]: Ending SSL/TLS
May 23 20:25:08 nadia webcit[615]: Ending SSL/TLS
May 23 20:25:08 nadia webcit[615]: Ending SSL/TLS
May 23 20:25:08 nadia citserver[9135]: citserver[9135]: Stack frame #0: /usr/loc
al/citadel/citserver(+0x36f0a) [0x55c470fbaf0a]
May 23 20:25:08 nadia citserver[9135]: Stack frame #0: /usr/local/citadel/citser
ver(+0x36f0a) [0x55c470fbaf0a]
May 23 20:25:08 nadia citserver[9135]: citserver[9135]: Stack frame #1: /lib/x86
_64-linux-gnu/libpthread.so.0(+0x14420) [0x7f7bb7bbb420]
May 23 20:25:08 nadia citserver[9135]: Stack frame #1: /lib/x86_64-linux-gnu/lib
pthread.so.0(+0x14420) [0x7f7bb7bbb420]
May 23 20:25:08 nadia citserver[9135]: citserver[9135]: Stack frame #2: /lib/x86
_64-linux-gnu/libc.so.6(+0x188947) [0x7f7bb7992947]
May 23 20:25:08 nadia citserver[9135]: Stack frame #2: /lib/x86_64-linux-gnu/lib
c.so.6(+0x188947) [0x7f7bb7992947]
May 23 20:25:08 nadia citserver[9135]: citserver[9135]: Stack frame #3: /lib/x86
_64-linux-gnu/libc.so.6(+0x78d15) [0x7f7bb7882d15]
May 23 20:25:08 nadia citserver[9135]: Stack frame #3: /lib/x86_64-linux-gnu/lib
c.so.6(+0x78d15) [0x7f7bb7882d15]
May 23 20:25:08 nadia citserver[9135]: citserver[9135]: Stack frame #4: /lib/x86
_64-linux-gnu/libc.so.6(+0x8bf9a) [0x7f7bb7895f9a]
May 23 20:25:08 nadia citserver[9135]: Stack frame #4: /lib/x86_64-linux-gnu/lib
c.so.6(+0x8bf9a) [0x7f7bb7895f9a]
May 23 20:25:08 nadia citserver[9135]: citserver[9135]: Stack frame #5: /usr/loc
al/citadel/citserver(+0x1d0a1) [0x55c470fa10a1]
May 23 20:25:08 nadia citserver[9135]: Stack frame #5: /usr/local/citadel/citser
ver(+0x1d0a1) [0x55c470fa10a1]
May 23 20:25:08 nadia citserver[9135]: citserver[9135]: Stack frame #6: /usr/loc
al/citadel/citserver(+0x473c9) [0x55c470fcb3c9]
May 23 20:25:08 nadia citserver[9135]: Stack frame #6: /usr/local/citadel/citser
ver(+0x473c9) [0x55c470fcb3c9]
May 23 20:25:08 nadia citserver[9135]: citserver[9135]: Stack frame #7: /usr/loc
al/citadel/citserver(+0x3876e) [0x55c470fbc76e]
May 23 20:25:08 nadia citserver[9135]: citserver[9135]: Stack frame #8: /usr/loc
al/citadel/citserver(+0x3e843) [0x55c470fc2843]
May 23 20:25:08 nadia citserver[9135]: citserver[9135]: Stack frame #9: /usr/loc
al/citadel/citserver(+0x3810f) [0x55c470fbc10f]
May 23 20:25:08 nadia citserver[9135]: citserver[9135]: Stack frame #10: /lib/x8
6_64-linux-gnu/libpthread.so.0(+0x8609) [0x7f7bb7baf609]
May 23 20:25:08 nadia citserver[9135]: citserver[9135]: Stack frame #11: /lib/x8
6_64-linux-gnu/libc.so.6(clone+0x43) [0x7f7bb7929353]
May 23 20:25:08 nadia citserver[9135]: Stack frame #7: /usr/local/citadel/citser
ver(+0x3876e) [0x55c470fbc76e]
May 23 20:25:08 nadia citserver[9135]: Stack frame #8: /usr/local/citadel/citser
ver(+0x3e843) [0x55c470fc2843]
May 23 20:25:08 nadia citserver[9135]: Stack frame #9: /usr/local/citadel/citser
ver(+0x3810f) [0x55c470fbc10f]
May 23 20:25:08 nadia citserver[9135]: Stack frame #10: /lib/x86_64-linux-gnu/li
bpthread.so.0(+0x8609) [0x7f7bb7baf609]
May 23 20:25:08 nadia citserver[9135]: Stack frame #11: /lib/x86_64-linux-gnu/li
bc.so.6(clone+0x43) [0x7f7bb7929353]
May 23 20:25:08 nadia webcit[615]: webcit[615]: StrBuf_ServGetln(): Server conne
ction broken: Success
May 23 20:25:08 nadia webcit[615]: StrBuf_ServGetln(): Server connection broken:
 Success
May 23 20:25:08 nadia webcit[615]: webcit[615]: HTTP: 200 [0.171787] GET /image?
name=hello
May 23 20:25:08 nadia webcit[615]: webcit[615]: Ending SSL/TLS
May 23 20:25:08 nadia webcit[615]: HTTP: 200 [0.171787] GET /image?name=hello
May 23 20:25:08 nadia webcit[615]: Ending SSL/TLS
May 23 20:25:08 nadia systemd[1]: citadel.service: Main process exited, code=kil
led, status=11/SEGV
May 23 20:25:08 nadia systemd[1]: citadel.service: Failed with result 'signal'.
May 23 20:25:09 nadia webcit[615]: webcit[615]: SSL_accept (sock=7) success, usi
ng TLS_AES_128_GCM_SHA256 on TLSv1.3 (128 of 128 bits)
May 23 20:25:09 nadia webcit[615]: SSL_accept (sock=7) success, using TLS_AES_12
8_GCM_SHA256 on TLSv1.3 (128 of 128 bits)
May 23 20:25:09 nadia webcit[615]: webcit[615]: Ending SSL/TLS
May 23 20:25:09 nadia webcit[615]: Ending SSL/TLS
May 23 20:25:09 nadia webcit[615]: webcit[615]: SSL_accept (sock=10) success, us
ing TLS_AES_128_GCM_SHA256 on TLSv1.3 (128 of 128 bits)
May 23 20:25:09 nadia webcit[615]: webcit[615]: SSL_accept (sock=9) success, usi
ng TLS_AES_128_GCM_SHA256 on TLSv1.3 (128 of 128 bits)
May 23 20:25:09 nadia webcit[615]: SSL_accept (sock=10) success, using TLS_AES_1
28_GCM_SHA256 on TLSv1.3 (128 of 128 bits)
May 23 20:25:09 nadia webcit[615]: SSL_accept (sock=9) success, using TLS_AES_12
8_GCM_SHA256 on TLSv1.3 (128 of 128 bits)
May 23 20:25:09 nadia webcit[615]: webcit[615]: Ending SSL/TLS
May 23 20:25:09 nadia webcit[615]: webcit[615]: Ending SSL/TLS
May 23 20:25:09 nadia webcit[615]: Ending SSL/TLS
May 23 20:25:09 nadia webcit[615]: Ending SSL/TLS
May 23 20:25:09 nadia systemd[1]: citadel.service: Scheduled restart job, restar
t counter is at 2.
May 23 20:25:09 nadia systemd[1]: Stopped Citadel Server.

Citserver has not crashed since my last message after installing the newer version, even with Webcit up.  Not sure what changed.

The upgrade to 1103 worked for me.

Thanks