I am running macOS Catalina 10.15.7 with NoMachine 7.0.211. I also have the same problem. It won’t let me authenticate (I am putting in the right password for my administrative account) and so I can’t shut down the server. I only want to use the client feature on this laptop.
I don’t know if this is relevant, but as with any time on Catalina that authentication is requested, the full user name shows up. My full user name has a comma in it:
Lastname, Firstname I.
Mac logins are synchronized with Active Directory using Enterprise Connect.