Because other premium members have takent the slot already. Premium members receive an extra slot but that means the whole group of members receive it, not every member.
If you cannot log in after clearing the cookies and trying a different browser I honestly have no idea how else to help you, everyone else is able to log in so the problem is on your side, not ours. Sorry.
Go here: You are not allowed to view links. Register or Login to view. Type your email (darkside2252@gmail.com) - go to your inbox, follow instructions. Log in.
but as i know your rule says that changing your players nick is forbbiden that is why i ask to make sure, like your players nick will be saved with another server nick.