Hi,
@WJ :
The issue comes from a strange setup of your checkout.
You've created a menu item "registration form" of the type "URL" with a URL you apparently would like for the checkout and then selected that menu item in the "force menu on checkout".
I would rather recommend that you change the type of that menu item to "HikaShop checkout", change its alias to "naar-de-ideal-kassa" and then it should work properly with the URL
www.vishandelschilder.nl/naar-de-ideal-kassa
It's actually a miracle that the way you set it up didn't cause any issue until now
@mariussport :
Similarily, on lebasport.nl your "force menu on checkout" setting is set to the "Bestelling en Login" menu item which isn't either a hikashop type menu item but a Joomla article. Please try to use a HikaShop checkout menu item type for that "force a menu on checkout" setting. Now, I wasn't able to reproduce the issue on your website as you still have the 4.0.3, but I highly suspect that this is the problem.
In fact, with your feedbacks, I've noted on my end to add some warning in the HikaShop configuration for such situation.