I've tried a few different approaches to fix this - iisreset, disable SSL on PowerShell virtdir, enable KerbAuth.dll native on PowerShell virtdir, ensuring a valid cert is applied to the exchange backend site in IIS. I have also updated to the most recent Update. I even tried the Exchange Management Trouble shooter which fails https://gallery.technet.microsoft.com/exchange-management-b9d918b1

We only use exchange 2013 for O365 Purposes so its not heavily used I literally cant think of anything else. The problem I think has happened in July 7th update I think. I cant seem to connect to the admin portal EPC or even the exchange toolbox.

Start-Transcript c:\MySession.txt -Append

VERBOSE: Connecting to Servername
New-PSSession : [Server Name] Connecting to remote server Server Namel failed with the
following error message : The WinRM client cannot process the request. It cannot determine the content type of the
HTTP response from the destination computer. The content type is absent or invalid. For more information, see the
about_Remote_Troubleshooting Help topic.
At line:1 char:1
+ New-PSSession -ConnectionURI "$connectionUri" -ConfigurationName Microsoft.Excha ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo         : OpenError: (System.Manageme....RemoteRunspace:RemoteRunspace) [New-PSSession], PSRemotin
  gTransportException
    + FullyQualifiedErrorId : -2144108297,PSSessionOpenFailed
VERBOSE: Connecting to Server Name.
New-PSSession : [Server Name] Connecting to remote server [Server Name] failed with the
following error message : The WinRM client cannot process the request. It cannot determine the content type of the
HTTP response from the destination computer. The content type is absent or invalid. For more information, see the
about_Remote_Troubleshooting Help topic.
At line:1 char:1
+ New-PSSession -ConnectionURI "$connectionUri" -ConfigurationName Microsoft.Excha ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo         : OpenError: (System.Manageme....RemoteRunspace:RemoteRunspace) [New-PSSession], PSRemotin
  gTransportException
    + FullyQualifiedErrorId : -2144108297,PSSessionOpenFailed
VERBOSE: Connecting to [Server Name] 
New-PSSession : [Server Name]  Connecting to remote server [Server Name] failed with the
following error message : The WinRM client cannot process the request. It cannot determine the content type of the
HTTP response from the destination computer. The content type is absent or invalid. For more information, see the
about_Remote_Troubleshooting Help topic.
At line:1 char:1
+ New-PSSession -ConnectionURI "$connectionUri" -ConfigurationName Microsoft.Excha ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo         : OpenError: (System.Manageme....RemoteRunspace:RemoteRunspace) [New-PSSession], PSRemotin
  gTransportException
    + FullyQualifiedErrorId : -2144108297,PSSessionOpenFailed
VERBOSE: Connecting to [Server Name] 
New-PSSession : [Server Name] Connecting to remote server [Server Name] failed with the
following error message : The WinRM client cannot process the request. It cannot determine the content type of the
HTTP response from the destination computer. The content type is absent or invalid. For more information, see the
about_Remote_Troubleshooting Help topic.
At line:1 char:1
+ New-PSSession -ConnectionURI "$connectionUri" -ConfigurationName Microsoft.Excha ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo         : OpenError: (System.Manageme....RemoteRunspace:RemoteRunspace) [New-PSSession], PSRemotin
  gTransportException
    + FullyQualifiedErrorId : -2144108297,PSSessionOpenFailed
Failed to connect to an Exchange server in the current site.
Enter the server FQDN where you want to connect.: