If you get the above error, it usually indicates that one or more of remote desktop sessions to your VPS were interrupted or not properly logged off.

This can be fixed easily by connecting directly to the console session of the VPS.

To do this, open up remote desktop as normal, but when entering the IP address (or server name) specify the admin option.

So, for example, instead of entering "79.170.111.222" you would enter "79.170.111.222 /admin" 

Now when you log in you should be connected directly to the server's console session.

To kill off the old disconnected RDP sessions, use CTRL-ALT-END and click Task Manager, select the User tab, and log off the other users.

Your users should now be able to log in normally next time.

Note that on some old RDP clients you have to use /console instead of /admin