It seems to be hosted by these people: https://agdsn.de/sipa/news/?locale=en

Basically, it's a non-profit club by university students from Dresden, Germany, which provide all kinds of IT services for other students, but also happen to use their server capacity to mirror the F-Droid repo.

I don't think that website contains any information about the mirror itself, aside from the generic page that any F-Droid repo provides: https://ftp.agdsn.de/fdroid/repo/

But well, there's not really much to say about a mirror repo anyways. It just mirrors the contents of the official F-Droid repo, so it contains exactly the same apps. It exists to help reduce the load on the official F-Droid server. Via checksums, it can also be ensured that the mirror server offers the correct files for download, so you don't even have to trust it.

Normally, if a mirror goes down or is unreachable, your F-Droid client should just select another mirror and download from there. I don't know why that didn't work here...