Does it do that for every app? That only happens to me when I try to download, for example, a hdpi theme on ICS. even then it doesnt happen all of the time.
If what jonny said doesn't work reboot in to recovery and wipe cache and dalvik cache and reboot and wipe cache for market and force stop market then it should work
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No. all you need to do is delete your google account and re-add it. then reboot and you'll be able to download apps. this used to happen to me all the time when I still my OV.