Cannot add a site with a 302 redirect

My site is https://www.eressea.de/ and when I tried adding a web proof for it, I got the dreaded "That domain isn't loading. Are you running a site there?" message. This time though, it was not because of an incomplete certificate chain (I've been here before), but it seems to have been caused by the 302 redirect that the index.php page makes - it redirects to another URL on the same server, based on the browser's Accept-Language header. Once I removed that logic,, I was able to add the site, then add the proof, and after it was accepted, add the logic back in.

Reproduce the web-proof add-site flow with https://www.eressea.de/ or another site whose index.php returns a same-server 302 based on Accept-Language. Trace the site-loading validation used during proof setup; done means a valid same-server redirect no longer prevents the site or proof from being added.

Written by the indexing model from the issue text.