Why can't I use my graphic digital signature?

In Acrobat XI Mac, I have used a graphic signature on the same field in the same self-created form for years. Suddenly, I cannot use it--the fill-and-sign tools dialog is grayed out except for the certificate radio button. Acrobat used to just let me click on the sign field and up popped the graphic file dialog. When I tried it today, it demanded I enter detailed info... and I couldn't get out of that path for several fields, eventually cancelling the action. I thought. Now it appears a digital signature certificate file exists, but I don't want or need it. There is no significant transaction or security required here--it's just a convenience so I don't have to waster time, toner, paper, etc. printing, manually signing, then re-scanning dozens of times. How do I get my graphic signature to work again?

Thank you,

Karen