Since the control to prevent the installation of two different products on Windows doesn't work as expected, the user is able to install a different NoMachine package over a previous installation of another package, e.g. install a Personal Edition over a client. During this process, permissions of the user's home directory are altered and as a consequence, the user is no longer able to log-in to the Windows system.

As a workaround, run Windows in safe mode and open a cmd console as administrator.

In the cmd console execute this command to restore proper pemissions:

icacls "C:\Users\<username>" /grant <username>:(OI)(CI)(F) /t

Then reboot Windows normally.