I'm not sure if this applies to your situation or not, but I just spent 12 hours trying to get a return filed that had the same error.

My call to Intuit resulted in them scratching their heads and unable to figure out what was wrong as well.

I had resolved myself to just doing the return in ProConnect Tax Online because there was nothing else I could do and needed to file the return. The only reason I ended up going back to ProSeries was because DEPRECIATION DID NOT TIE OUT BETWEEN PROSERIES AND PROCONNECT. I was going to have to re-send the return for signature at the S-Corp level, re-send all K-1s, Correct all Shareholder returns... I was getting very frustrated.

My significant other came in from working and we were going through the issues. As a last resort, I showed him the 3 attachments to the return because we were checking the total characters in the path of the upload to ensure they didn't exceed the 250 limit imposed by Windows. The file length was fine BUT the SIZE of the files was enormous. They were pages that had signatures that my clients took a photo up and sent to me. We compressed the files and viola... returns processed and were successfully submitted.