From SoftwareDistribution.log on the WSUS server:
2013-07-26 22:46:53.550 UTC Error WsusService.8 WebServiceCommunicationHelper.VerifyServerCertificate The verification of the certificate chain failed.
at Microsoft.UpdateServices.Internal.WebServiceCommunicationHelper.VerifyServerCertificate(Object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
at System.Net.Security.RemoteCertificateValidationCallback.Invoke(Object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
at System.Net.ServerCertValidationCallback.Callback(Object state)
at System.Threading.ExecutionContext.runTryCode(Object userData)
at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Net.ServerCertValidationCallback.Invoke(Object request, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
at System.Net.Security.SecureChannel.VerifyRemoteCertificate(RemoteCertValidationCallback remoteCertValidationCallback)
at System.Net.Security.SslState.CompleteHandshake()
at System.Net.Security.SslState.CheckCompletionBeforeNextReceive(ProtocolToken message, AsyncProtocolRequest asyncRequest)
at System.Net.Security.SslState.ProcessReceivedBlob(Byte[] buffer, Int32 count, AsyncProtocolRequest asyncRequest)
at System.Net.Security.SslState.StartReceiveBlob(Byte[] buffer, AsyncProtocolRequest asyncRequest)
at System.Net.Security.SslState.ProcessReceivedBlob(Byte[] buffer, Int32 count, AsyncProtocolRequest asyncRequest)
at System.Net.Security.SslState.StartReceiveBlob(Byte[] buffer, AsyncProtocolRequest asyncRequest)
at System.Net.Security.SslState.ProcessReceivedBlob(Byte[] buffer, Int32 count, AsyncProtocolRequest asyncRequest)
at System.Net.Security.SslState.StartReceiveBlob(Byte[] buffer, AsyncProtocolRequest asyncRequest)
at System.Net.Security.SslState.ForceAuthentication(Boolean receiveFirst, Byte[] buffer, AsyncProtocolRequest asyncRequest)
at System.Net.Security.SslState.ProcessAuthentication(LazyAsyncResult lazyResult)
at System.Threading.ExecutionContext.runTryCode(Object userData)
at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Net.TlsStream.ProcessAuthentication(LazyAsyncResult result)
at System.Net.TlsStream.Write(Byte[] buffer, Int32 offset, Int32 size)
at System.Net.PooledStream.Write(Byte[] buffer, Int32 offset, Int32 size)
at System.Net.ConnectStream.WriteHeaders(Boolean async)
at System.Net.HttpWebRequest.EndSubmitRequest()
at System.Net.HttpWebRequest.GetResponse()
at System.Web.Services.Protocols.WebClientProtocol.GetWebResponse(WebRequest request)
at System.Web.Services.Protocols.HttpWebClientProtocol.GetWebResponse(WebRequest request)
at System.Web.Services.Protocols.SoapHttpClientProtocol.Invoke(String methodName, Object[] parameters)
at Microsoft.UpdateServices.Internal.DssAuthWebService.DssAuthWebService.Ping(Int32 pingLevel)
at Microsoft.UpdateServices.Internal.HealthMonitoring.HmtWebServices.CheckDssAuthWebService(EventLoggingType type, HealthEventLogger logger)
at Microsoft.UpdateServices.Internal.HealthMonitoring.HealthMonitoringTasks.ExecuteSubtask(HealthMonitoringSubtask subtask, EventLoggingType type, HealthEventLogger logger)
at Microsoft.UpdateServices.Internal.HealthMonitoring.HealthMonitoringTasks.Execute(EventLoggingType type)
at Microsoft.UpdateServices.Internal.HealthMonitoring.HealthMonitoringThreadManager.Execute(Boolean waitIfNecessary, EventLoggingType loggingType)
at Microsoft.UpdateServices.Internal.HealthMonitoring.HealthMonitoringThreadManager.PeriodicTask()
at Microsoft.UpdateServices.Internal.HealthMonitoring.PeriodicTaskExecuter.ThreadStart()
at System.Threading.ExecutionContext.runTryCode(Object userData)
at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart()