Follow-up: we resolved a resource leak bug affecting Mac OS X, all versions, that MAY have caused your problem.
For sure it solves this crash:
https://www.nomachine.com/TR12K04134
We can’t be sure it also solves your problem. There is the theoretical possibility it does, since, after a while, the server started to behave inconsistently. Anyway we were able to reproduce it here only once and since updating the code we have not reproduced it anymore, so there is some hope. Another user experiencing the same problem received the fix yesterday. He has not provided his feedback yet. The new code should ship as part of the 4.1 on next Monday.