Hello,
I got my sites accessible with proxying now (thanks @sjr and @Erisa ) but stumbled on another issue. Calibre-web doesn’t seem to like to be proxied, and the proposed workaround not only is based on the deprecated page rules, but it also isn’t working for me. I keep getting logged off, and most of the images don’t load when I manage to get the books page.
I have calibre-web running on my server on port 2583, and the origin rule is working, allowing me to load the site.
ETA: it can be fixed by a configuration change in calibre-web, so please ignore. See here.
My apologies