Hi,

it is definitely no memory or open files issue. I stopped all
processes and tried to start one of the faulty instances by itself,
without luck.

Actually, there are core files. I don't know whether they say
something interesting, but I just saw that each start attempt produced
a core file.

This is the backtrace of one of them:

root@kammel:~# gdb /usr/sbin/kamailio
/var/cores/core_kamailio_24894_6_110_1464180675
GNU gdb (GDB) 7.4.1-debian
Copyright (C) 2012 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /usr/sbin/kamailio...Reading symbols from
/usr/lib/debug/.build-id/0e/a2651a480b84540fc37d4d0d640aa3a4db078c.debug...done.
done.
[New LWP 24894]

warning: Can't read pathname for load map: Input/output error.
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Core was generated by `/usr/sbin/kamailio -f
/etc/kamailio/kamailio_sip_lb_sipconnect_de_v6_1.cfg -P /'.
Program terminated with signal 6, Aborted.
#0  0x00007fef1812b125 in raise () from /lib/x86_64-linux-gnu/libc.so.6
(gdb) bt
#0  0x00007fef1812b125 in raise () from /lib/x86_64-linux-gnu/libc.so.6
#1  0x00007fef1812e3a0 in abort () from /lib/x86_64-linux-gnu/libc.so.6
#2  0x00000000004ab7d9 in sig_alarm_abort (signo=<optimized out>) at main.c:649
#3  <signal handler called>
#4  0x00007fef181d3b77 in syscall () from /lib/x86_64-linux-gnu/libc.so.6
#5  0x00007fef0fc69d50 in futex_get (lock=<optimized out>) at
../../parser/../mem/../futexlock.h:108
#6  mod_destroy () at rtpengine.c:1970
#7  0x0000000000580092 in destroy_modules () at sr_module.c:811
#8  0x00000000004ac407 in cleanup (show_status=show_status@entry=1) at
main.c:524
#9  0x00000000004acf4f in shutdown_children (show_status=1, sig=15) at
main.c:666
#10 0x00000000004adac7 in handle_sigs () at main.c:758
#11 0x00000000004b22e6 in main_loop () at main.c:1733
#12 0x0000000000427e2b in main (argc=<optimized out>, argv=<optimized
out>) at main.c:2616

Best Regards,
Sebastian