I just posted a similar problem.

FYI - ProSeries generates the 760C as part of the VA return.  So it's not the 760 that is not being attached. It appears to be some other problem like the one I posted.  In my case, ProSeries wants a supporting statement attached as a PDF.  I attached it and ProSeries is still slinging off the error message. I'm going to disable error checking and move on.