This worked for me (from HDINFO's comment above):
*) Go into the object properties of your signature field.
*) On the 'Signature' tab, IF you have the box checked for "Lock fields after signing", but the drop-down box is blank,
then you can either recreate your fields collection, or you can uncheck that box.
Once I recreated my collection and re-assigned it to the signature field, everything worked perfectly!