Appears the issue is related to DCI Tables.
All segfault backtraces mention DCTable.
I then ran netxmsd with -D9 option for testing and found several prepare statements for a DCI Table prior to the crash - but no completed sync.
I had such an issue in the past with a template for Intel Modular servers. When I could not determine the underlying cause, I "simply" switched all tables in all templates to instances and the system had been stable since. Appears I missed one: a single template (for a Netgear ReadyNAS) with a single device bound to it still had DCI tables in use.
It appears the NAS performed an automatic firmware upgrade to version 6.6.1 and within an hour NetXMS started crashing. It is not clear if the template for the NAS had not been applied until after the upgrade (auto-apply rules) or if the upgrade simply triggered something in the SNMP responses that caused the crashes of the server.
Since I disabled the DCI Tables, the server has not crashed (around 90 minutes of uptime now, over 6 times as long as the longest interval between crashes before that). It looks like there is some kind of issue within the DCI table code that can lead to segfaults and server crashes.