2017867 - SE03: Object cannot be assigned to custom package

Modification of package name for an object in SE03 transaction is aborted with error TO124:

Object <OBJECT_TYPE> <OBJECT_NAME> cannot be assigned to package <CUSTOM_NAMESPACE>
Message no. TO124

Diagnosis
The object <OBJECT_TYPE> <OBJECT_NAME> belongs to namespace <SAP_NAMESPACE>. The object cannot be assigned to development class <CUSTOM_NAMESPACE>, since it does not belong to the same namespace.

System Response
The function is terminated.

Procedure
Assign the object to a package whose name begins with <SAP_NAMESPACE>.

If you want to create the object as a test object, you can also assign it to a local package (begins with $) or a test package (begins with T).

RSWBO052 TO124 custom own customer development class package namespace TO 124  $TMP , KBA , BC-CTS-ORG , Workbench/Customizing Organizer , Problem