hi jjk, thank you for your quick answer! Very appreciated! However, I again removed all other extensions, incl. Template, helix, seo, sp page builder, etc. incl. all other payment plugins. I sent you some screenshots about the back- and frontend. Unfortunately, I have to unroll all these steps (can't leave the site at this stage). But as soon as I get rid of the paypal payment field, the warning just disappeared. So it has to be something about that paypal plugin. Is there any way, I can uninstall and install it separately? If yes, do you have a download link for me, please?
thank you again for keeping up with your ideas!
Have a nice time!