We strongly recommend you update to 5.1.24 and check whether the problem disappears. We haven’t been able to reproduce it on the 5.1.22.
If you still encounter the problem with the latest version, please check that you haven’t disabled “Start automatic services at startup” in nxclient monitor (click on nxclient tray icon, choose “Show the server status”, and then – “Server preferences” – it should say “Running” status)?
If you see that it is enabled already, it is possible that the node is crashing. In that case, we need to investigate. Can you check whether you have nxtrace.log in log directory? If you do, please run debug (check article linked below), restart nxserver and try to reproduce the problem. If you were able to repeat issue, please send all logs from server side to forum[at]nomachine[dot]com or attach them here (size is restricted). Instructions about debug and collecting logs you can find there: https://www.nomachine.com/DT04M00076.