What do light grey song titles mean in iTunes? When song 5 is done playing, it automatically skips over to song 16. The songs in between play just fine when started manually but are not part of the queue when playing the album. Why? What do these grey song titles mean?

I do have iTunes Match, but these songs are all downloaded to my computer.
Switching to the "songs" view showed the problem: they were unchecked for some reason (no idea why). And because this checkmark wasn't visible in the "artists" view (which I always use) I never noticed this before.

Anyway, selecting all songs -> check selection solved the problem!