We needed to install an antispam plugin to stop the daily flood of 30-50 spam messages. Sometimes the filter is a bit too harsh, so accidentally some email or ip is categorized as from a spammer. Unfortunately this seems to be happening during your registration. The antispam-plugin uses various spammer databases and we have no control over them at all.
We advice you to scan your computer with an updated virusscanner as your computer might be part of a spambots network.
You can try from another computer and/or with another emailadress.
If that also does not work, use the contactform and mail us which username you would like so we can make an account for you.