Action manager customizing does not support processing mode &1

What causes this issue?

The function module 'EWA_S_AMDEF_MAINTAIN' for Customizing the action
manager was called up with an invalid processing mode.
The invalid processing mode was '&V1&'.
This is a typical problem error.

System Response

The program is terminated with a runtime error.

How to fix this error?

Correct your program or contact SAP, specifying this message if this
concerns a program delivered by SAP.

Error message extract from SAP system. Copyright SAP SE.