Apparently, we found the problem. We compared the clean install to the upgraded one, step by step. When we switched off plagiarism detection (enableplagiarism - had been active in our 2.2 version), everything works just fine and as usual. I don't really see how plagiarism detection could possibly affect frontpage functions - but that's what solved the problem. Looks like a bug to me... Can anyone confirm that?
@Howard: Website administration was visible in the menu, the cursor changed on mouseover, but didn't respond to clicks. We haven't tried debugging, server logs looked normal. Thanks!