What is a "exception.code:31008" ?

Summary:

What is the reaon of "exception.code:31008"?

After a requisition approver submits an approval email against the approval request notification email, return the error email as below screenshot.

If someone encounterd same error or happen to know some clues, I would like advices..

NOTE : In header japanese meaning like "Response against this notification were not processed".

Content (please ensure you mask any confidential information):

Other approver can approve by email. Only some of approvers encounterd this error.

When a requisition approver submit an approval email against the approval request notification email,

Version (include the version you are using, if applicable):