You could always just reinstall the client in a new location. You'll lose whatever unit you were currently working on, but at least you'll be able to get back to crunching...
Just keep the old folder around... Maybe it's something server side. Try re-launching the old client every once in a while to see if it tries to upload.
I don't know much about Folding@Home as I've only done maybe 5 units on it, so this is just my two cents.