Hello!

Similar issue but with different symptoms. 

kamailio 4.4.7 (tried to use db_mongodb module from 5.1.2)
usrloc is configured to use db_mode=1 and db_insert_null=1.

MongoDB 3.6.3
mongo-c-driver was tested 1.9.2 and 1.9.4.

location table contains 300 records.

The issue was not reproduced since February when we started a new feature. Now it is reproduced constantly, but on some machines.

Logs:

 1(13958) DEBUG: usrloc [udomain.c:313]: dbrow2info(): non-local socket <all.com>...ignoring
 1(13958) CRITICAL: usrloc [udomain.c:476]: preload_udomain(): empty username record in table location...skipping
 1(13958) CRITICAL: usrloc [udomain.c:256]: dbrow2info(): bad callid
 1(13958) ERROR: usrloc [udomain.c:484]: preload_udomain(): skipping record for agent in table location
 1(13958) CRITICAL: usrloc [udomain.c:476]: preload_udomain(): empty username record in table location...skipping
 1(13958) CRITICAL: usrloc [udomain.c:231]: dbrow2info(): bad contact
 1(13958) ERROR: usrloc [udomain.c:484]: preload_udomain(): skipping record for cflags in table location
 1(13958) DEBUG: usrloc [udomain.c:313]: dbrow2info(): non-local socket <��keepalive>...ignoring
 1(13958) WARNING: usrloc [../../ut.h:717]: shm_str_dup(): shm_str_dup fallback; dup called for src->s == NULL or src->len < 0
 1(13958) WARNING: usrloc [../../ut.h:731]: shm_str_dup(): shm_str_dup fallback; skip memcpy for src->s == NULL
 1(13958) CRITICAL: usrloc [udomain.c:476]: preload_udomain(): empty username record in table location...skipping
 1(13958) CRITICAL: usrloc [udomain.c:231]: dbrow2info(): bad contact
 1(13958) ERROR: usrloc [udomain.c:484]: preload_udomain(): skipping record for cseq in table location
 1(13958) CRITICAL: usrloc [udomain.c:231]: dbrow2info(): bad contact
 1(13958) ERROR: usrloc [udomain.c:484]: preload_udomain(): skipping record for 5.17.xxx in table location
 1(13958) CRITICAL: usrloc [udomain.c:231]: dbrow2info(): bad contact
 1(13958) ERROR: usrloc [udomain.c:484]: preload_udomain(): skipping record for .17.x in table location
 1(13958) CRITICAL: usrloc [udomain.c:476]: preload_udomain(): empty username record in table location...skipping
 1(13958) DEBUG: usrloc [udomain.c:313]: dbrow2info(): non-local socket <nce>...ignoring
 1(13958) WARNING: usrloc [../../ut.h:717]: shm_str_dup(): shm_str_dup fallback; dup called for src->s == NULL or src->len < 0
 1(13958) WARNING: usrloc [../../ut.h:731]: shm_str_dup(): shm_str_dup fallback; skip memcpy for src->s == NULL
 1(13958) CRITICAL: usrloc [udomain.c:476]: preload_udomain(): empty username record in table location...skipping
 1(13958) CRITICAL: usrloc [udomain.c:476]: preload_udomain(): empty username record in table location...skipping
 1(13958) CRITICAL: usrloc [udomain.c:476]: preload_udomain(): empty username record in table location...skipping
 1(13958) DEBUG: usrloc [udomain.c:313]: dbrow2info(): non-local socket <c-89-5ada654b-35a2-42>...ignoring
 1(13958) DEBUG: usrloc [udomain.c:313]: dbrow2info(): non-local socket <>...ignoring
 1(13958) CRITICAL: usrloc [udomain.c:231]: dbrow2info(): bad contact
 1(13958) ERROR: usrloc [udomain.c:484]: preload_udomain(): skipping record for 38.146:5360 in table location
 1(13958) DEBUG: usrloc [udomain.c:313]: dbrow2info(): non-local socket <ified>...ignoring
 1(13958) ERROR: usrloc [ucontact.c:94]: new_ucontact(): no ruid for aor: lQ.com@5.17.60@8/Ղusername
 1(13958) ERROR: usrloc [urecord.c:149]: mem_insert_ucontact(): failed to create new contact
 1(13958) ERROR: usrloc [udomain.c:517]: preload_udomain(): inserting contact failed

 1(13958) ERROR: usrloc [ul_mod.c:457]: child_init(): child(1): failed to preload domain 'location'
 1(13958) ERROR: <core> [sr_module.c:923]: init_mod_child(): Error while initializing module usrloc (/usr/ecsp/lib64/kamailio/modules/usrloc.so)
 1(13958) ERROR: <core> [pt.c:329]: fork_process(): init_child failed for process 1, pid 13958, "udp receiver child=0 sock=5.17.138.137:5060 (5.17.138.137:5060)"
 1(13958) CRITICAL: <core> [main.c:1615]: main_loop(): Cannot fork
 0(13956) ALERT: <core> [main.c:740]: handle_sigs(): child process 13958 exited normally, status=255
 0(13956) INFO: <core> [main.c:768]: handle_sigs(): terminating due to SIGCHLD
 0(13956) DEBUG: <core> [main.c:770]: handle_sigs(): terminating due to SIGCHLD

Could you please help us to resolve the issue?
Thank you in advance.
Best regards
Konstantin