Thanks, @ThomasIx. The Cart sends the current URL to the back-end for each page load. This is so that when items are added to the cart, the server knows which page and gallery the items belong to.
For some reason, your server is objecting to a URL like this being passed as a parameter:
https://www.foto-ix.de/galleries/01-schleppjagden/2024/05-jagd-verden/?page=2
In particular, the section ?page=2.
Do you know if your site has had any recent security modules installed? mod_security is a common module that could be incorrectly detecting a request like this as a security threat.
A similar situation came up last week on the forums: Search yields error
I’ll look into changing the Cart so that it omits everything from the ? when sending the current page to the back end. I’ll need to work out whether that part of the URL is of importance to the back-end of the Cart.