When you unchecked the box for U.S. Government interest, did you then click Continue? If you don't click Continue it will not save the change. If you go back and edit the 1099-DIV again, is that box still unchecked? Unchecking that box should have eliminated the error about box 1a.
Do you have more than one Form 1099-DIV with box 1a blank? If so, make sure the U.S. Government interest box is unchecked for all of them.
