This is the backtrace of 2 core-dumps i got just recently (both with the same timestamp).
Nothing obvious that hits my eyes (at least no NULL pointer). Maybe you can see more in it.

Backtrace #1:
===========
[New LWP 99122]
[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 -P /var/run/kamailio/kamailio.pid -f /etc/kamailio/kamailio.'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x00007fca0d45e996 in t_should_relay_response (Trans=0x7fca067b2a60, new_code=401, branch=0, should_store=0x7ffff47edd44, should_relay=0x7ffff47edd40, cancel_data=0x7ffff47edf30, reply=0x7fca0d8fd020) at t_reply.c:1282
1282    t_reply.c: No such file or directory.
(gdb) bt
#0  0x00007fca0d45e996 in t_should_relay_response (Trans=0x7fca067b2a60, new_code=401, branch=0, should_store=0x7ffff47edd44, should_relay=0x7ffff47edd40, cancel_data=0x7ffff47edf30, reply=0x7fca0d8fd020) at t_reply.c:1282
#1  0x00007fca0d46317f in relay_reply (t=0x7fca067b2a60, p_msg=0x7fca0d8fd020, branch=0, msg_status=401, cancel_data=0x7ffff47edf30, do_put_on_wait=1) at t_reply.c:1786
#2  0x00007fca0d469154 in reply_received (p_msg=0x7fca0d8fd020) at t_reply.c:2537
#3  0x000055e77686fbe3 in do_forward_reply (msg=0x7fca0d8fd020, mode=0) at core/forward.c:747
#4  0x000055e776871a00 in forward_reply (msg=0x7fca0d8fd020) at core/forward.c:852
#5  0x000055e7768bd4e7 in receive_msg (
    buf=0x55e776da0520 <buf> "SIP/2.0 401 Unauthorized\r\nVia: SIP/2.0/UDP 172.17.217.10;rport=5060;received=172.17.217.10;branch=z9hG4bKe4bf.c46a843afa3ff97f5b20df53926515a2.0;i=62\r\nVia: SIP/2.0/TCP 195.225.36.194:1797;rport=40047;"..., len=736,
    rcv_info=0x7ffff47ee4e0) at core/receive.c:364
#6  0x000055e7767ce4fe in udp_rcv_loop () at core/udp_server.c:554
#7  0x000055e77673a15d in main_loop () at main.c:1619
#8  0x000055e7767421fe in main (argc=13, argv=0x7ffff47eeb98) at main.c:2638

Backtrace #2:
===========
[New LWP 99120]
[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 -P /var/run/kamailio/kamailio.pid -f /etc/kamailio/kamailio.'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x00007fca0d45e996 in t_should_relay_response (Trans=0x7fca067da3c0, new_code=200, branch=0, should_store=0x7ffff47edd44, should_relay=0x7ffff47edd40, cancel_data=0x7ffff47edf30, reply=0x7fca0d8fd020) at t_reply.c:1282
1282    t_reply.c: No such file or directory.
(gdb) bt
#0  0x00007fca0d45e996 in t_should_relay_response (Trans=0x7fca067da3c0, new_code=200, branch=0, should_store=0x7ffff47edd44, should_relay=0x7ffff47edd40, cancel_data=0x7ffff47edf30, reply=0x7fca0d8fd020) at t_reply.c:1282
#1  0x00007fca0d46317f in relay_reply (t=0x7fca067da3c0, p_msg=0x7fca0d8fd020, branch=0, msg_status=200, cancel_data=0x7ffff47edf30, do_put_on_wait=1) at t_reply.c:1786
#2  0x00007fca0d469154 in reply_received (p_msg=0x7fca0d8fd020) at t_reply.c:2537
#3  0x000055e77686fbe3 in do_forward_reply (msg=0x7fca0d8fd020, mode=0) at core/forward.c:747
#4  0x000055e776871a00 in forward_reply (msg=0x7fca0d8fd020) at core/forward.c:852
#5  0x000055e7768bd4e7 in receive_msg (
    buf=0x55e776da0520 <buf> "SIP/2.0 200 OK\r\nVia: SIP/2.0/UDP 172.17.217.10;rport=5060;received=172.17.217.10;branch=z9hG4bK3494.0e37cca5019fe3c95285a3464d0eaa5e.0;i=b3\r\nVia: SIP/2.0/TCP 192.168.0.102:53360;rport=53360;received=9"..., len=638,
    rcv_info=0x7ffff47ee4e0) at core/receive.c:364
#6  0x000055e7767ce4fe in udp_rcv_loop () at core/udp_server.c:554
#7  0x000055e77673a15d in main_loop () at main.c:1619
#8  0x000055e7767421fe in main (argc=13, argv=0x7ffff47eeb98) at main.c:2638

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