Hello Juergen,
in fact it is a very interesting problem, because today We test second time and didn't find any problems.
Well, let's see, in testing We use Firefox and hosting from Hostgator
1. In Global configuration I set SMTP Host
2. We create new menu item Registration form (Menu Item Type: Registration form)
3. Public this menu item in main menu
4. Complete all registration fields and press registration button
5. on mail box I received new email from site and press on link
6. Attachement image
7. I sucessfully login on site
Will be very nice if you can send us Access to site via contact form and we test this issue on your site
Best Regards,
OrdaSoft team