I may not be able to help - I have never seen this problem before - but we can try to work on this together. I need however a bit more information: Which version of Acrobat are you using, and on what operating system?
Did you create the digital signature? Is the signature still valid (meaning, not expired)? What happens when you create a new signature, does that work?
Karl Heinz Kremer
PDF Acrobatics Without a Net
PDF Software Development, Training and More...
http://www.khkonsulting.com