New order activity cannot be determined after quantity change

What causes this issue?

An internal error has occurred.
The quantity of an order was changed. Therefore, it receives a new
activity. You have to determine this (ACTID) to reschedule the order,
for example.

System Response

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

How to fix this error?

Inform SAP.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.