• In Studio, VDA report Registered, but power state is Unknown, Starting, Off etc.  
• Users might not be able to get a session, since Citrix will not broker a connection to a machine that has power state Off or Starting.  
• The VDA are fine.  Looking directly in the Hyper-V host (versus SCVMM host), the consoles show up.  You can can RDP to the VDA.
• In the SCVMM console, the Hyper-V hosts have red x's and the status is 'Host Not Responding'.
• In the Jobs --> History in SCVMM console, this error (or similar) is logged:  
• 

The machine name redacted is the Hyper-V host.
The message says:  The Windows Remote Management (WinRM) client on the VMM server cannot process the request.  A computer policy does not allow the delegation of the user credentails to the target computer computer.domain.com (this computer name will be the name of a Hyper-V host). 
WinRM URL: [http://computer.domain.com:5985], Verb [IINVOKE], Method: [GetVersion], Resource: [http://schemas.microsoft.com/wbem/wsman/1/wmi/root/scvmm/AgentManagement]
Use gpedit.msc and look at the following policy: Computer Configuration -> Administrative Templates -> System -> Credentials Delegation -> Allow Delegating Fresh Credentials. Verify that it is enabled and configured with an SPN appropriate for the target computer. For example, for a target computer name myserver.domain.com, the SPN can be one of the following: WSMAN/myserver.domain.com OR WSMAN/*.domain.com or WSMAN/*
ID: 20553
Details: Unknown error (0x803381a3)

The solution is in the error message. 
1) Run RSOP to make sure that there is no Domain level policy for Computer Configuration -> Administrative Templates -> System -> Credentials Delegation -> Allow Delegating Fresh Credentials
2) Run Gpedit.mcs and configure the policy. 
You can wait for a time until the status refreshes in SCVMM.  The the state of VDA's will be updated shortly without restarting any services. 
 

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Problem Cause

In this case, there was some unknown action had reconfigured the policy in the machine that hosts the System Center and changed it to un-configured. 
The Hosting connection in Studio shows no error because the System Center is not down.

The host connection in Studio queries the SCVMM System Center host. In this case the SCVMM was not down, but only could not provide any information. With this condition, the state of the guest VM's becomes unknown to SCVMM and in turn, the data provided to Citrix Broker becomes stale.