I had this same issue. None of these suggestions worked.
Eventually I figured out the file size for the signature page I attached was too large. It must have been timing out during upload. My scanned signature page was around 20 mb. I reduced it to around 400 kb and then it worked.
The error message was completely useless and they should check the attachment file size and set a limit or warning.
It also took me forever to be able to login to my account just to post up this solution.
I'm glad TurboTax exists, but please do better, Intuit.
