Using Adobe Acrobat Pro DC and initially had problems that elicited this message:
"At least one signature has problems. Please fill out the following form."
This message was eliminated by doing the following:
Once the PDF form was closed and re-opened, the first message was gone but replaced by:
"Signed and all signatures are valid, but with unsigned changes after the last signature. Please fill
out the following form."
Now how do I get rid of this second, unwanted message?
Sorry for the delay in response to your query and apologize for the inconvenience caused. Please refer and try the suggestions from the following KB docs which discusses the similar issue Adobe Digital Signature Validation issue
You may also refer to Validating digital signatures, Adobe Acrobat
Please check for any pending updates of Adobe Acrobat Pro DC from Help>Check for updates. Restart the machine after the installation of the updates.
Let us know how it goes.