PayPal redirect fail

Hello,

It’s about a website I created for a client. I have entered my client’s PayPal email address in your PlugIn and switched off the sandbox mode. However, it doesn’t seem to be able to connect to PayPal, it always comes up with the error message “Unfortunately, a system error has occurred. Please try again later”, in my case in German, as the page is in German. Is there anything you can do to help me? The URL is: https://konzerte-liberme.de/events/geburtstagskonzert

Greetings, Mariya Kupisok.

The page I need help with: [log in to see the link]