Okay, so I think I finally have this figured out, but not resolved yet.
Just now, I was working on some other projects and ran into an issue with the disk space in the temporary folder filling up because someone was uploading a world over 3GB.
So needless to say I think it's dying because the temp folder is running out of disk space. I'm going to increase this when I have a free second and this should allow for larger worlds to be uploaded.
I'll post an update in a few days when I've had a chance to test this potential solution.
EDIT: I apologize to the person who spent all that time uploading a 3GB+ world just now and lost all that time. Your timing is impeccable however as it allowed me to track down the issue...