FAS not working, event 104 generated on VDA:

[S104] Identity Assertion Logon failed.  Failed to connect to Federated Authentication Service: UserCredentialService [Address: fas.domain][Index: 0] [Error: The caller was not authenticated by the service. 
Server stack trace: 
   at System.ServiceModel.Security.IssuanceTokenProviderBase`1.DoNegotiation(TimeSpan timeout)
   at System.ServiceModel.Security.SspiNegotiationTokenProvider.OnOpen(TimeSpan timeout)
   at System.ServiceModel.Security.WrapperSecurityCommunicationObject.OnOpen(TimeSpan timeout)
   at System.ServiceModel.Channels.CommunicationObject.Open(TimeSpan timeout)
   at System.ServiceModel.Security.CommunicationObjectSecurityTokenProvider.Open(TimeSpan timeout)
   at System.ServiceModel.Security.SymmetricSecurityProtocol.OnOpen(TimeSpan timeout)
   at System.ServiceModel.Security.WrapperSecurityCommunicationObject.OnOpen(TimeSpan timeout)
   at System.ServiceModel.Channels.CommunicationObject.Open(TimeSpan timeout)
   at System.ServiceModel.Channels.SecurityChannelFactory`1.ClientSecurityChannel`1.OnOpen(TimeSpan timeout)
   at System.ServiceModel.Channels.CommunicationObject.Open(TimeSpan timeout)
   at System.ServiceModel.Security.SecuritySessionSecurityTokenProvider.DoOperation(SecuritySessionOperation operation, EndpointAddress target, Uri via, SecurityToken currentToken, TimeSpan timeout)
   at System.ServiceModel.Security.SecuritySessionSecurityTokenProvider.GetTokenCore(TimeSpan timeout)
   at System.IdentityModel.Selectors.SecurityTokenProvider.GetToken(TimeSpan timeout)
   at System.ServiceModel.Security.SecuritySessionClientSettings`1.ClientSecuritySessionChannel.OnOpen(TimeSpan timeout)
   at System.ServiceModel.Channels.CommunicationObject.Open(TimeSpan timeout)
   at System.ServiceModel.Channels.ServiceChannel.OnOpen(TimeSpan timeout)
   at System.ServiceModel.Channels.CommunicationObject.Open(TimeSpan timeout)
   at System.ServiceModel.Channels.ServiceChannel.CallOpenOnce.System.ServiceModel.Channels.ServiceChannel.ICallOnce.Call(ServiceChannel channel, TimeSpan timeout)
   at System.ServiceModel.Channels.ServiceChannel.CallOnceManager.CallOnce(TimeSpan timeout, CallOnceManager cascade)
   at System.ServiceModel.Channels.ServiceChannel.EnsureOpened(TimeSpan timeout)
   at System.ServiceModel.Channels.ServiceChannel.Call(String action, Boolean oneway, ProxyOperationRuntime operation, Object[] ins, Object[] outs, TimeSpan timeout)
   at System.ServiceModel.Channels.ServiceChannelProxy.InvokeService(IMethodCallMessage methodCall, ProxyOperationRuntime operation)
   at System.ServiceModel.Channels.ServiceChannelProxy.Invoke(IMessage message)

Exception rethrown at [0]: 
   at System.Runtime.Remoting.Proxies.RealProxy.HandleReturnMessage(IMessage reqMsg, IMessage retMsg)
   at System.Runtime.Remoting.Proxies.RealProxy.PrivateInvoke(MessageData& msgData, Int32 type)
   at Citrix.Authentication.UserCredentialServices.IConvertCredentials.GetConvertCredentialsVersionNumber(Int32 clientVersion)
   at Citrix.Authentication.IdentityAssertion.HdxCredentialSelector.<>c__DisplayClass24_0.<QueryLogonMethod>b__0()]

Add VDA Computer group to have access on FAS server through 'Access this computer from the Network' policy under 'Computer Configuration\Windows Settings\Security Settings\Local Policies\User Rights Assignment'.

By default, 'Everyone' and 'Users' are added to the 'Access this computer from the Network' policy. In some organizations these settings are modified to limit access for security. FAS users, VDAs and StoreFront servers (for on-prem) need access to the FAS server for FAS to work correctly. These three groups should be added to the policy if Users and Everyone groups are removed.

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Problem Cause

If VDAs do not have access to FAS server, FAS service fails to authenticate the VDA and returns HTTP 500 for Certificate requests.