After re-installing a PE server on a CA Data Protection Gateway errors similar to the ones below start to appear in the activity log.
Sep 13, 2018 04:32:57 AM NT AUTHORITY\SYSTEM E0928 The server failed during initialization/policy update. Exception was 'Obtained unexpected value from policy'.
Sep 13, 2018 04:32:59 AM NT AUTHORITY\SYSTEM I0917 Loaded policy 'UserMaster/UnknownInternalSender', version 2.4932.1.
CA Data Protection 15.0 Gateway Server with Policy Engine server parented to another Gateway.
This is caused by a a mismatch in the expected and actual policy ID numbers
Parenting the Gateway server directly with the CMS forces the Gateway to resync all objects and this addresses the problem.