I am currently running Kubuntu 21.10 and install NoMachine free version 7.7.4 (latest).
It works great with KDE plasma.
Then i installed a xmonad and test it on physical computer and it works.
Then when try to connect the physical display from NoMachine I got
“Cannot detect any display running. Do you want NoMachine to create a new display and proceed to connect to the desktop?”
When I connect to SDDM from NoMachine and choose Xmonad as desktop and login I got black screen with mouse cursor but can do any things.
But as here mentioned https://knowledgebase.nomachine.com/TR06R09773 this should be already fixed in version 7.0.208
I didn’t find anythings in log files under /usr/NX/var/log .
I am sure it has worked before in earlier versions.