NX nomachine - user limit error
Even though you have the "free" Linux version, to the best of my knowledge it is limited to two concurrent users.
You might want to investigate FreeNX which is built on the NoMachine source. There was a new version released at the end of last year, which I haven't tried yet, that may allow more users. There used to be an issue with FreeNX server and the nomachine client, but that may have been fixed in the latest release.