Hi Christiane,
Ah, that is even a PHP error it seems.
I am sorry to say, I prefer not to have logins on websites for support. At the support forum at wordpress.org I would get into problems with that.
Is it possible for you to check the error log of PHP? Or alternatively, to enable debug settings for displaying errors?
See
https://wordpress.org/support/article/debugging-in-wordpress/
Regards,
Marcel