Thank you so very much Rikard!!!

WP Events+ this was absolutely the case. We do not use it anymore, as we run EventOn …
So I just deactivated WP Events+ and the Stripe is gone. But still very strange, that WP Events+ suddenly does this!
And I tested your Code in Quick CSS and this did the job as well. Just for the case, if there is someone else who maybe has the same problem …

Best Regards from Switzerland
Claude Michael Merz