I had this issue while performing a Centralized Upgrade in a Client's Production environment.
The main reason for this error was unknown at that time.
Restarting the Orion servers WILL NOT RESOLVE this issue.
So, to overcome this issue, I had to perform certain troubleshooting steps in the Client's environment.
After carefully analyzing the issue, it got cleared to me.
The issue was that the SolarWinds Administrator Service was missing on the Main Polling Engine and the Main Failover Polling Engine. ( Which was a very uncommon thing. )
Because of that reason, it could not contact the Main Polling Engine when I tried to run the Centralized Upgrade.
So, I installed the SolarWinds Administrator Service on BOTH servers and started the upgrade process again.
Voila!
It worked!
Afterwards, I completed the upgrade process smoothly.
I just thought of sharing this experience which might be helpful for the community.
