Might be corrupt database files. Have you tossed them and let Avid re-index?
Also, check to make sure you don’t have any illegal characters in the filenames of your source material: hyphens, spaces, dashes, parenthesis, colons, etc…
I’ve also heard this may come up if your media drive is full, but I’m sure you already checked that.
What exactly were you doing when this cropped up?