I just found a trigger for this, and it seems to have nothing to do with codecs.
My config: MaLa 1.0.4.5. xVid 1.1.2 with ffdshow. (MaLa is happy with this)
The problem: No start video, and no screensaver (video blinks a black screen a moment, or pic shows "no files found" message). Videos in layouts play fine.
I noticed that deleting the mala.ini file and starting clean made the problem disappear. From there, it was just a small matter of setting up DiffMerge and testing one diff at a time between a known "bad" and known "good" mala.ini. Narrowed it down to the repeatable trigger, at least for me. Switching 1 to 0 makes both video and pic screensavers work, and switching back to 1 breaks it again:
[Mouse]
UseMouse=0
See attachment for the GUI equivalent. In other words, if you're using a mouse (or trackball, or spinner) in MaLa, there's a better than crap chance that your screensaver is stuffed.
And yes, I am posting this just after midnight (my time) on New Years Eve. All those whistles and fireworks and drunk people I can hear outside are all cheering because I've discovered a repeatable testcase I'm sure.