Membership plugin not accepting any Usernames

Hi,
I've set up the Membership plugin at my site http://jessheimtorg.no. You can see the various subscribtions here: http://jessheimtorg.no/registrering/

My problem is that the system is not approving ANY new Usernames. It says that the username is not valid. (I've already translated it to Norwegian, so I don't remenber the exact wording...)

Any ideas why?

Br.
FiGu