There was no endpoint listening at http://xxxxxx.com:8021/DiagnosticService that could accept the message

PROBLEM: 

There was no endpoint listening at http://xxxxxx.com:8021/DiagnosticService that could accept the message.

SOLUTION:  

• Check the ports and make sure they are correct.  
• Make sure there is not a conflict with the port number 

In this case, try changing the port number from 8021 to 8001

Add TCP port inbound firewall rules for ports 8000-8012 to Central and site database hosts