destination breaks form alteration

I have custom profile fields which the user is expected to fill out during registration. This works fine when the user is visiting the standard registration page. (here: http://www.ms-hns.de/user/register)
But when he's redirected to the page and gets some argument like a new destination, it won't work. (see: http://www.ms-hns.de/user/register?destination=node%2F630 or even just http://www.ms-hns.de/user/register?t)

With this bug users can register without filling out the required fields which breaks the functionality of the module.