Webpc-passthrough.php missing?

Hi, I had the WebP converter running for several years without problems. I had configured my .htaccess and checked the delivery of webp images, it worked flawlessly.

However, somewhere in the past few months, unbeknownst to me, it must have stopped working. I just got aware of it through a popup window. The settings now claim there’s an error in my server configuration (rewrites_not_working)… I can’t really comprehend that… why all the sudden? Also, rewrites still work well for permanent-links..

I tried everything that I could think of: deactivate every other plugin (esp. WP-Rocket and Smush), check my .htacces, CHMOD of all files and directories, file ownership etc etc. still, nothing…

Anyway, I followed the advice and switched the delivery mode to “passthrough”- and got another error. When I tried to find the reason I realized that the file webpc-passthrough.php cannot be found anywhere on my server. Certainly not in the ‘upload’ directory.. which is, by the way, not the place where my image files are being stored. I have my media files in the folder “/media”, as configured in the “media”-section of the wordpress-settings.

so, now I’m out of my wits:
– why did webp suddenly stop working?
– where do I get the file webpc-passthrough.php? Will this method work?
– how can I tell webp converter the right directory of my mediathek?
– how else can I check the server configuration?
.. please help!