Just to make sure: You are using a paid licensed version, not the 'trial' version right? And if you check in Server - License Management, it does show the proper number of domains you purchased?
If all that is showing good, then the next thing I would try is to go to Clients - Limits, uncheckmark the 'unlimited' and specify a hard number (10 or 20), save those changes. Then try creating another domain. If that works, then you can go back again and try setting it back to unlimited.
Either that or I would use phpMyAdmin on the server and directly check the 'psa' database tables to correct the problem.
I have seen some instances where the database field has an erroneous number entered when a limit is set for 'unlimited', this should clear up that type issue.