Unable to Enroll Symantec Endpoint Protection Manager (SEPM) to cloud. Getting Error- 'Internal Server Error'. The error occurs when they enter the enrollment token and hit ‘Enroll Symantec Endpoint Protection Manager’.

As per semapisrv_log we get below exception-

2017-12-06 12:18:31,472 [http-apr-8446-exec-9] ERROR c.s.s.s.c.e.h.GlobalControllerExceptionHandler - EXCEPTION:  
com.symantec.sepm.server.module.common.service.exception.IllegalStateServiceException: Symantec Endpoint Protection Manager cannot connect to the cloud portal. Check the network connection and try again.

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2017-12-06 12:18:34,432 [http-apr-8446-exec-7] ERROR c.s.s.s.c.e.h.GlobalControllerExceptionHandler - EXCEPTION: usea1.r3.securitycloud.symantec.com 
java.net.UnknownHostException: usea1.r3.securitycloud.symantec.com

SEPM Version: 14 and above

SEPM Server OS: Windows 2012 R2 Server, Windows 2016

Computer is behind a proxy that does not allow the enrollment to complete.

Exclude the following URLs in proxy exception list 

aws.amazon.com
ncs-spoc-int1.norton.com
usea1.r3.securitycloud.symantec.com/*
*.s3.amazonaws.com

For more information regarding exclusions refer to the following KB article  
https://techdocs.broadcom.com/us/en/symantec-security-software/endpoint-security-and-management/endpoint-security/sescloud/Getting-Started/urls-to-whitelist-for-v129099891-d4155e9710.html