The same error happened to me when I changed the name of subform but I didn't reedit collection of fields that should be locked after signing.
Check everything when you change field or form names.
Hope this helps.
I am also facing the same issue. next time when attempting, the system is not reading DSC Key. and asking to check if token interface is properly configured.
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!