I tried the solution below and it works :
You have run out of available user CALs.
Go to QMC > System > Licenses
Click on the QV Server. On the General tab (on the right) you will see how many CALs have been assigned, and how many you have in you license.
Go to the Assigned CALs tab. Now go through the list and see which user's haven't logged in for a long time (you must decide how long). Now you can delete those assigned CAL's. Remember to click apply after each delete.
There should be some space now for new users.
If no CALs can be deleted, you will have to purchase more licenes.