I used to get this problem on the old machine under KDe3,
I never got a solution as it was something intermittent.
On LXDE beta i did manage to have the same problem you describe after a great amount of switching between the LX and Main installs (both beta's currently installed).
The only thing i can think of was a rather odd file transfer i did, copied a remaster into my separate "var" partition prior to burning.
This seems a most unlikely cause of the complaint.
Sorry i cannot offer a solution.