The underlying connection was closed: A connection that was expected to be kept alive was closed by the server.

Unable to connect to the remote server -–> System.Net.Sockets.SocketException: No connection could be made because the target machine actively refused it

A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond.

XenCenter log

 debug log %appdata%\Citrix\XenCenter\logs\XenCenter.log

 C:\Documents and Settings\<user name>\Application Data\Citrix\XenCenter\logs

 C:\Users\<user name>\AppData\Roaming\Citrix\XenCenter\logs