Receiving email error for invoice approvals, but it still approves

We have a user who has approved multiple invoices over email but is receiving this error. However, the invoice is showing approved in Oracle. Has anyone seen this before or know of a quick fix?

"Your response to this notification was not processed. exception.code:31009 exception.type: ERROR exception.severity: 2 exception.name: Incoming message content is invalid. exception.description: Incoming message could not be processed due to invalid content. exception.fix: Ensure that the response message is not corrupted: The encrypted response should begin and end with "[[NID]]"."