Inbound category &1 not known in target system &2; message refused

What causes this issue?

The &SYST-MSGV2& target system marks the &SYST-MSGV1& inbound
category as unknown and rejects the report(s).

System Response

No processing takes place in the target system.

How to fix this error?

Check whether the transferred inbound category &SYST-MSGV1& corresponds
to an inbound category defined in the &SYST-MSGV2& target system.
If necessary, correct the coding for your function module for FS-CD exit
V038, or adjust the settings or Customizing in the target system.

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.