I just found out, that the CAPTCHA function seemed to play a role in the appearance of the blank page. When I set the configuration so that new registrants should need to solve the MATH CAPTCHA, I see a blank page. When I deactivate Spam-protection, registering works.

But I would like to have a SPAM protection for new members of the forum.

Does anybody has an idea?

Thanks for help,
Arthur Dent