I’m getting the same error. I’m running Ubuntu 19.10.
Unable to run nxserver, even from the /etc/NX directory.
sessions.<user> only had the value 1 in the file. Deleting that file made no difference.
Problem just started this morning.
I’m wondering if a complete uninstall and reinstall will do the trick. It’s a person server, so I don’t have to worry about other people being inconvenienced.