Thanks to Stefan Hirtbach who just send me a critical issue about the MANIAC.net WebAdmin: If WebAdmin (or any other way) is used to assign keys to an user, the call of a stored procedure within the MANIAC.net fails:
maniac_sp_getProductKeyForUser checks for the number of keys = 1. If a user has more than one key assigned to him, he can order unlimited keys due to this condition. Just change the condition to >= 1 wich fixes the problem.
A new package of the MANIAC.net WebAdmin will be available soon, wich contains a fixed version of the stored procedure.