Picasa does not show .mp4 videos

Recently i ran into an issue where Picasa did not show all my .mp4 videos anymore, though I was sure that this  worked before.

Searching the web pointed out that Quicktime must be installed and Picasa be set to show Quicktime Videos (.MOV) in Tools-->Options..., but that was already the case.

After playing around with the Picasa and Quicktime settings, I had Quicktime reinstall its default file associations and: Bingo! Picasa showed these files again.