Error adding a site or opening an existing site in Essentials Manager

Hello I had to reconfigure the ArcGIS Server Account (same  user /password) pointing to Essentials and now I get an error whe opening or adding a site in  Essentials Manager.

 

 "An error occurred while processing your request.The cause of this error is unknown.

 

"http://DOMAINNAME/Geocortex/Essentials/"INSTANCENAME""/RestManager/Error?aspxerrorpath=/Geocortex/Essentials/"INSTANCENAME"/RestManager/Account/Logon"

 

The error when "Adding a new site" in Essentials Manager site wizard is:  "{"redirect":"/Geocortex/Essentials/"INSTANCENAME"/RestManager/Account/Logon?ReturnUrl=http%253a%252f%252fDOMAINNAME%252fGeocortex%252fEssentials%252f"INSTANCENAME"

 

I checked  Site/ application/ folder permissions and they all seem fine. The HTML5 Viewer and ArcGIS Server  are both OK. The problem is with Essentails Manager. Any leads as to how to resolve this issue are appreciated.

 

Oscar