I started getting this problem earlier today after the n900 reset itself while gPodder was downloading. I reset afterwards when I got to work and noticed that user had become read-only and started getting this error along with a lot of error messages about being out of storage (file manager reported several hundred MBs free). I tried several different things that might have caused the problems without luck.

Re-flashed the device, both errors are still occurring and I am unable to restore a backup because of the out of memory error.

It almost seems like permissions issue affecting tracker as the media player isn't picking up on files. Could a permissions issue persist through a re-flash, and could that make tracker and calendar break?