After adding an MD5 Blacklist Policy in Advanced Threat Protection (ATP) one or more Symantec Endpoint Protection Managers (SEPMs) display Authentication or Connection Errors as their Status.
Multiple 12.1.x SEPMs are configured in Settings > Global Settings> Endpoint Detection and Response > Symantec Endpoint Protection Manager (SEPM) Web Servers
Replication is enabled between all SEPMs
The issue is resolved in SEP14.
If you cannot upgrade to SEP14 yet, a workaround is available:
You may also contact Symantec Technical Support for assistance with steps 2 and 3 above if you have many Client Groups and SEPM Domains