Cannot persist key mapping for contact persons &1 and &2; see long text

What causes this issue?

You want to transfer a plant with contact person data for customers
and/or suppliers.

System Response

The system has created the business partner for the contact person.
However, the system cannot persist the key mapping of contact person
&V1& in the SAP ERP system and contact person &V2& in the SAP S/4HANA
system.

How to fix this error?

Check the detailed messages and analyze why the key mapping is not
persisted.
Use transaction
<DS:TRAN.MDG_KM_MAINTAIN>MDG_KM_MAINTAIN</> to maintain the key mapping
information manually.

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.