Could you please let me know the version of Adobe Acrobat and the Operating System installed on your machine? This you can tell by navigating to Help -> About Adobe Acrobat.
Hi, it is versin 9.0
Are you adding a digital signature field to the document or are you signing an existing signature field? If an existing field, is it the last unsigned field in the document? Is the document an XFA form (created with LiveCycle Designer) by any chance?