System error: Process instruction & in phase & not found

What causes this issue?

The process instruction &V2& for operation &V1& of the process order
does not exist any more due to an internal error.

System Response

The processing step cannot be signed.
The status of the PI sheet cannot be set to "complete".

How to fix this error?

INCLUDE 'NA_SYSVERW' OBJECT DOKU ID TX LANGUAGE E

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.