if you are getting this error on Exchange 2013 CAS below is the fix. I have no idea what went wrong but both my CAS servers (lab environment stopped working). I was lucky enough to find the similar post for Exchange 2010 and soon after

The WinRM client received an HTTP server error status (500), but the remote service did not include any
other information about the cause of the failure.

FIX

SERVER MANAGER—–Features——Add Features——-WinRM IIS Extension —- you are done!