EndpointCentral System manager and Remote control not working

EndpointCentral System manager and Remote control not working

We have endpoint central UEMS up and running for over a year now, and everything has been working well
Until we recently couldn't get any configurations done using System manager (under Tools), we are presented with the error : Agent has been disconnected.Please close window and re-initiate  or Session Expired! Close the windows and try again depending on with component we use.
We tried everything to resolve this, the ports are open (it worked before as mentioned) and the telnet to the server on port 8443 is working properly, the troubleshooting on the agent is all green.
we thought it would get fixed with new updates, but nothing happened, it got worse when now even the remote control doesn't work (both HTML5 and ActivX)
Please resolve this issue for us

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