System error: Table T020 incorrectly maintained &

What causes this issue?

There is an incorrect entry in the control table for the flow of
transactions in the Financial Accounting (FI) area.
This table is of the 'S' type (system table) and is maintained by SAP
only.

System Response

The processing cannot be continued.

How to fix this error?

There is probably an error in the delivery system or with the release
changeover. Contact SAP. You can then correct the incorrect entry
yourself using the the corresponding table maintenance authorization.

Error message extract from SAP system. Copyright SAP SE.