What happens if you create a pdf of a blank piece of paper and attach it - does the error clear?

You could go to the ProSeries list of bugs to see if is a known issue, but even after asking for such a list for the last century, Intuit does not want to supply such a critter. 

If the blank pdf doesn't solve the problem your only other option is to call to support and at least that way you will know if it is a problem they are working on or if they are shocked to find such an issue.