I had something similar to this on my droid x. Try this:
Backup your android.secure folder (in case you need anything in it in the future), and then delete that folder on your sd card.
Try market. If it works, that means one of the files in that android.secure folder was corrupt. I'm pretty sure market replaces the files in it when it needs them, but if not, you'll have your backup in cae something goes awry down the road.