I came across this error during an Exchange 2010 Unified Messaging deployment. The EMC would not connect:
"The attempt to connect to http://server.domain.com/PowerShell using "Kerberos" authentication failed: connecting to remote server failed with the following error message : The WinRM client cannot complete the operation within the time specified. Check if the machine name is valid and is reachable over the network and firewall exception for Windows Remote Management service is enabled. For more information, see the about_Remote_Troubleshooting Help topic."
There are various blog posts on the internet around how to fix the connectivity problem to the server, but in this case the server EMC was pointing to had been decommissioned properly and was no longer listed in AD. EMS would connect fine to a different working server.
In my case I had to take two actions to fixed it.
Under C:\users\<specific user>\AppData\Roaming\Microsoft\MMC\Exchange Management Console\ there is a config file. Delete it
In the registry under HKCU\Software\Microsoft\Exchangeserver\v14\AdminTools\NodeStructureSettings delete the value NodeStructureSettings
Only after both of these are done, did the EMC correctly rediscover an active Exchange 2010 server. If one or there other is done, the incorrect server information is retained.
Also as a workaround I could add a “new” Exchange forest and connect, but I wanted to actually fix the issue.