Incorrect function module & called up

What causes this issue?

The function module &v1&, which was called for the application input,
was not able to process the IDoc. A possible cause is that the IDoc has
wrong message type or IDoc type.

How to fix this error?

Please check the message type assignment for the application function
module in the ALE customizing.

Error message extract from SAP system. Copyright SAP SE.