I had the same error and nothing in this thread worked for me. The issue for me ended up being my signed 8453-PE form.
I had originally scanned it at 1,200 dpi, saved as PDF, and attached it in TT. There were no errors or warnings advising there was an issue with the attachment, it said it was successful. When I was ready for the final step to transmit, I got hit with the Error: 2 We're experiencing Internet transmission problems...
After trying absolutely everything, I decided to delete/remove my signed 8453-PE PDF form from TT. I re-scanned it at 600 dpi, saved it as a new PDF, re-attached it in TT, & BAM SUCCESS! I think my original scan/file was just too big for TT. You'd think TT would have a warning or disclosure for something like this.
