Error while saving purchasing plant reassignments

What causes this issue?

The transaction was aborted after an error was detected.

System Response

Your changes will be lost if you attempt to save again or if you try to
shut down the application.

How to fix this error?

Contact your system administrator in order to retrieve your data.

Procedure for System Administrators

Purchasing plant assignment data is stored in internal table
t_mmsiteref_modif

Error message extract from SAP system. Copyright SAP SE.