Multiple users can not log on

I am running the latest version of a Lion server on a Mac Mini. I have a number of client machines. I am having no problems with logging on, accessing my home directories, shares etc. All users are set up with their home directories on the server. All the clients are running the latest mountain lion (one iMac, two macpros and one macbook).

However, I have recently been having a problem with logging on multiple users. If I have one user logged on one of the clients and then go to the login window (leaving that user logged on in the background) I can not log on another network user. It says that the user can not be logged on at that time. If I log out the first user on the client before trying to log on the second user I have no problems.

If I go to a second client and log any user on while I am still logged on the first client - I have no problems logging any user on.

All users are set to allow multiple log ons.

Interestingly, in the server app, when I log a user on, the user is shown twice in the file server tab under connected users with the same IP address. This is the case for all users that are logged on at the time on any of the clients.

Any ideas, suggestions etc would be welcome.

Regards,

Tim