Thread number would help.

I got it.

Using the normal "Expand Thread" button at the top works flawlessly.

If I click "First 100 posts" / "Last 50 posts" and then "Entire Thread", then 404. Same behavior with ALL threads in the archive.

All in all, my problem is solved, since the threads are there and well.

But maybe some code monkey wants to have a look at the button-linkage-stuff, I don't know, not my concern anyways...

I was aware of that bug, went to check, didn't see it. Another look and I realize it only happens to the links at the bottom of the page, not at the top. Template edited, pages rebuilt.