The step list editor was called with an invalid operation code (&)

What causes this issue?

A call to the function module BP_STEPLIST_EDITOR contains an incorrect
value for the STEPLIST_OPCODE parameter. The function module call is
incorrectly programmed.

System Response

Function terminated.

How to fix this error?

Contact SAP for assistance with this problem, as the function module
belongs to those that are reserved for internal SAP use only.

Error message extract from SAP system. Copyright SAP SE.