You can always try disabling NeatUpload if you think it is the culprit. Basically comment out anything that references NeatUpload in Web.config. Then you're back to regualr asp.net uploading.
This is an isolated incident. This thread is from Feb 2011 and no other reports since then. I stand by my previous suggestions on this thread. Most likely cause is disk space allocation limit reached.
I'm on vacation from now till Feb 19, so I hope this helps as I won't be here to help further until I get back.
Hope that helps,
Joe