patriciamandek
In template settings > Features tab > the Registration URL is set to: index.php?option=com_users&view=registration
On frontend, it will look to the menu item for user registration on your site and redirect to that page. But there was not this type of menu item page on your site so it keeps showing on current page Itemid=101 is homepage.
I checked backend of your site and there is an item called 'Anmelden' has type of user registration >> I enabled it and it redirects to this page correctly now.
This is default behaviour of Joomla.
Regards