I went to the user page and renamed him from Michael Müller to Michael Müller. Also in content there was an auto-generated (by a hook) draft page michael-muller that I now manually renamed to michael-mueller. I don’t think the draft page should be related to this at all, so I am starting to suspect there was some sort of encoding/unicode issue with the letter ü that maybe got fixed when I retyped the name??!
Sorry, I think I will be unable to reliably pinpoint the original issue now that it is fixed. So, all I can say is, thanks a lot @texnixe for staying with me and trying to hunt down this super weird problem!