I'm one of the developer of your forum software Phorum. We got a bug report from one of your new users having problems to register.
I don't think that's a problem of our Phorum software. I checked the source code and can't find any explication for this behavior.
You are using some kind of web security appliance?
P.S. I too wasn't able to register myself (my email address oliver at riesen dot org). I didn't receive the confirmation email.