Symptoms
AddPEMDomainForSubscription_API returns base-64 encoded error message:
Communication Failure: Error in method invocation.
Cause
Exception occurs because the domain with the same name is already present in OA
Resolution
Upgrade to OSA 7.0.x, since OSA 7.0 there is no exception for AddPEMDomainForSubscription_API for the domains already present in OA.