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