After reinstalling Extra Decoders Support and its dependencies (just Ogg Support might have been enough, but I didn't care to do it one-by-one) everything went back to normal and I can happily play .ogg and .flac. I think updating Tracker cleared some info about registered mime types (you can maybe confirm that by running "tracker-info path/to/file.ogg" and looking for contradictions, IIRC that helped me with the solution), so they had to be registered again. I think OMP has nothing to do with it. The playback is probably broken because MAFW is thinking that you're trying to feed it a non-audio file (and the reason is Tracker's incomplete mime info).
There's also one important thing regarding video bookmarks: it's quite different from the stock player. Nokia's way is to throw all videos into the video browser, no matter if they're stored locally or if they're just bookmarked addresses of video streams. In OMP, however, video bookmarks are accessible through the radio browser. Excluding "shuffle all", there are only 3 icons in the main window, so I think i makes more sense to divide media into {local music, local videos, remote music and videos} than {local music, local and remote videos, remote music}. Any objections? What do you think?
(BTW, Estel, why do I have a feeling that you never sleep? ).