Hi Carl,
I am wondering if you might have any suggestions on how to debug this and if you are logging any information in case a thread dies.
You could play around with trace() or prof:variables(), or activate debugging via -d or SET DEBUG ON.
We are running 8.5.2.
In BaseX 8.5.3, we have further improved thread-safety for fork-join queries. Could you give it a try?
Hope this helps Christian