That's a problem that I have been encountering as well. The error
message comes up when the upload process takes a long time to process
and the error page comes up when a certain amount of time has passed
without the new page coming up (I think its about a minute or so).
Next time this happens, check data sets to see if it actually
uploaded: then you don't have to worry about duplicate sets.
This problem is being worked on by the engineering crew.