Back to the future... I had 3 n900 (somehow now dead or unchargeable) having this issue for reasons I still can't identify.
I reflashed a new second hand n900 a few days ago and I re-enjoyed mediaplayer working for all my files again. But since yesterday, it is broken, again, and I can't figure out why.
The most frustrating thing is that I know I did nothing by myself to make this issue happen simply because I was sleeping between the moment it was working and the moment it was broken.

As far as I can tell, the problem is really similar to this *closed* bug:
https://bugs.maemo.org/show_bug.cgi?id=6823

I've tried a lot of things (restarting trackerd, killing mafw-*, reboots, run it with strace, etc.) without success. When trying to play any audio/video file, it still shows this error message (translated from french) "impossible to process operation", even trying to change the mediaplayer volume triggers this error.

If anybody here is still using his n900 and its mediaplayer, please give some feedback!

Note: openmediaplayer suffers of the same problems, an was working good when mediaplayer was working too.

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PS: my firmware is 20.2010.36-2