Thread 495 "$POLLERS/WRK" received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fff7258a700 (LWP 25764)]
Node::reconcileWithDuplicateNode (this=this@entry=0x7fffb038a000, node=0x7fffa56ed000) at node.cpp:3512
3512 node.cpp: No such file or directory.
(gdb) bt full
#0  Node::reconcileWithDuplicateNode (this=this@entry=0x7fffb038a000, node=0x7fffa56ed000) at node.cpp:3512
        i = 6
#1  0x00007ffff79d73c4 in Node::configurationPoll (this=0x7fffb038a000, poller=0x7fff7968bf80, session=0x0, rqId=0) at node.cpp:3230
        duplicateNode = 0x7fffa56ed000
        reason = L"Primary IP address 23.235.105.154 of node js1.northpointe01.mxu.acsalaska.net [43621] found on interface vlan.1997 of node js1.northpointe.cpe.lec.mgt [45588]", '\000' <repeats 152 times>...
        dcr = <optimized out>
        hypervisorType = L'\000' <repeats 31 times>
        hypervisorInfo = L"Primary IP address 23.235.105.154 of node js1.northpointe01.mxu.acsalaska.net [43621] found on interface vlan.1997 of node js1.northpointe.cpe.lec.mgt [45588]", '\000' <repeats 97 times>
        type = <optimized out>
        oldCapabilities = <optimized out>
        szBuffer = L'\000' <repeats 3494 times>...
        modified = 0
        __pollStartTime = 1575478327685
        __pollState = 0x7fffb038a768
#2  0x00007ffff796e3d2 in DataCollectionTarget::configurationPollWorkerEntry (this=0x7fffb038a000, poller=0x7fff7968bf80, session=0x0, rqId=0)
    at dctarget.cpp:1560
No locals.
#3  0x00007ffff793f621 in __ThreadPoolExecute_Wrapper_1<DataCollectionTarget, PollerInfo*> (arg=0x7fff79691420)
    at ../../../include/nms_threads.h:1346
        wd = 0x7fff79691420
#4  0x00007ffff698d337 in ProcessSerializedRequests (data=0x7fff796913c0) at tp.cpp:466
        rq = 0x7fff79691440
#5  0x00007ffff698d10e in WorkerThread (arg=0x7fff7a780390) at tp.cpp:186
        rq = 0x7fff796913e0
        waitTime = 256899072
        p = 0x7fff7dc00000
        q = 0x7fff7dc06000
        threadName = "$POLLERS/WRK\000\000\000"
#6  0x00007ffff4ec04a4 in start_thread (arg=0x7fff7258a700) at pthread_create.c:456
        __res = <optimized out>
        pd = 0x7fff7258a700
        now = <optimized out>
        unwind_buf = {cancel_jmp_buf = {{jmp_buf = {140735111800576, 7851966113022693045, 140736350368030, 140736350368031, 140735111536640, 3,
                -7851694838165582155, -7851990439467594059}, mask_was_saved = 0}}, priv = {pad = {0x0, 0x0, 0x0, 0x0}, data = {prev = 0x0,
              cleanup = 0x0, canceltype = 0}}}
        not_first_call = <optimized out>
---Type <return> to continue, or q <return> to quit---
        pagesize_m1 = <optimized out>
        sp = <optimized out>
        freesize = <optimized out>
        _PRETTY_FUNCTION_ = "start_thread"
#7  0x00007ffff3a15d0f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:97
No locals.
(gdb)

netxms-agent/stretch,now 3.1.242-2 amd64 [installed]
netxms-agent-java/stretch,now 3.1.242-2 amd64 [installed,automatic]
netxms-base/stretch,now 3.1.242-2 amd64 [installed,automatic]
netxms-client/stretch,now 3.1.242-2 amd64 [installed]
netxms-console/now 3.0.2258-1 amd64 [installed,local]
netxms-dbdrv-pgsql/stretch,now 3.1.242-2 amd64 [installed]
netxms-dbdrv-sqlite3/stretch,now 3.1.242-2 amd64 [installed,automatic]
netxms-release/now 1.5 all [installed,local]
netxms-server/stretch,now 3.1.242-2 amd64 [installed]

Segfaults whenever merge detection is enabled.

bt  full:

Thread 523 "$POLLERS/WRK" received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fff7282e700 (LWP 19692)]
0x00007ffff79bfbfe in Node::reconcileWithDuplicateNode(Node*) () from /usr/lib/x86_64-linux-gnu/libnxcore.so.31
(gdb) bt full
#0  0x00007ffff79bfbfe in Node::reconcileWithDuplicateNode(Node*) () from /usr/lib/x86_64-linux-gnu/libnxcore.so.31
No symbol table info available.
#1  0x00007ffff79d73c4 in Node::configurationPoll(PollerInfo*, ClientSession*, unsigned int) ()
  from /usr/lib/x86_64-linux-gnu/libnxcore.so.31
No symbol table info available.
#2  0x00007ffff796e3d2 in DataCollectionTarget::configurationPollWorkerEntry(PollerInfo*, ClientSession*, unsigned int) ()
  from /usr/lib/x86_64-linux-gnu/libnxcore.so.31
No symbol table info available.
#3  0x00007ffff793f621 in ?? () from /usr/lib/x86_64-linux-gnu/libnxcore.so.31
No symbol table info available.
#4  0x00007ffff698d337 in ?? () from /usr/lib/x86_64-linux-gnu/libnetxms.so.31
No symbol table info available.
#5  0x00007ffff698d10e in ?? () from /usr/lib/x86_64-linux-gnu/libnetxms.so.31
No symbol table info available.
#6  0x00007ffff4ec04a4 in start_thread (arg=0x7fff7282e700) at pthread_create.c:456
        __res = <optimized out>
        pd = 0x7fff7282e700
        now = <optimized out>
        unwind_buf = {cancel_jmp_buf = {{jmp_buf = {140735114569472, 6749483827744397886, 140736350368030, 140736350368031,
                140735114305536, 3, -6749736120991163842, -6749468538011329986}, mask_was_saved = 0}}, priv = {pad = {0x0, 0x0,
              0x0, 0x0}, data = {prev = 0x0, cleanup = 0x0, canceltype = 0}}}
        not_first_call = <optimized out>
        pagesize_m1 = <optimized out>
        sp = <optimized out>
        freesize = <optimized out>
        _PRETTY_FUNCTION_ = "start_thread"
#7  0x00007ffff3a15d0f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:97
No locals.
(gdb)

Does it segfaults all the time, or it happened only once?

If segfaults repeat, can you set DebugLevel=9 in server config, launch the server and attach the resulting log file?