alphanumeric, implies alpha & numeric.......so the 10 "digit" corporate number is probably not good enough to clear the error.

I do agree that this was not an issue a few months ago. in fact previously processed, submitted, error-free & accepted returns now have that diagnostic warning!

I disabled the e-file error warning and submitted.   Immediately after doing so I saw this across the screen: error occurred with background processing of EF Transmissions.

Now, waiting to see what happens. Not too confident.