what could I do? Downgrade to firmware 01.02.28 or install the actual firmware again 01.02.31?
I would try re-installing (manually) the .31 release and see if that helps. If not, go back to the .28 (you should be able to find a link to it here on the forum by typing in the f/w version number into the Search box (upper left).
It’s possible I suppose that something in the .31 version broke whatever it was that was fixed in the .28 version. Things sometimes happen that way. If you fiind that to be the case, post back and let us know; that would be something that the development team would want (and should) to know about so they can put it on their ‘to-do’ list.
As far as Rockbox, I’m not sure it even supports the ratings system. You could check at www.rockbox.org.