Thanks, but I really think my game/save was corrupted.
I tried everything for minutes (+ I watched a video to see, where I had to stand).
Then I loaded a save from another session and it immediately worked.
I've investigated the issue and it looks like somehow the game thought you'd already picked up the lice.
It's on a timer, so I wonder if something interrupted it when you played it through, so that it got as far as recording the lice, but not to the point where it actually put them in your inventory.
Glad you managed to get infested though (never thought I'd have to say that... again).