Issue in Global Services Monitor GSM

Dear all,

I'm facing a problem in Global Services Monitor.

The Resources pool contains 4 Management Servers; 2 old and 2 recently installed.
The GSM was installed and was working normally on the old Management Servers.
But, after increasing the number of management servers to be 4 instead of two, the problem appeared.
The GSM is firing alerts on the new MS's and their state are critical (old 2 servers are healthy).
The alert description is as below:

Global Service Monitor Modules: Failed to discover Global Service Monitor locations.
Failure step: 'Couldn't get the ACS endpoint from discovery service. SubscriptionId: 'a6846da0-e5d7-4bea-ab13-836d89364b60', OutsideInServiceBaseUri: 'https://gsm-prod.systemcenter.microsoft.com/''
Message: 'Could not establish trust relationship for the SSL/TLS secure channel with authority 'gsm-prod.systemcenter.microsoft.com'.'
Details: 'System.ServiceModel.Security.SecurityNegotiationException: Could not establish trust relationship for the SSL/TLS secure channel with authority 'gsm-prod.systemcenter.microsoft.com'. ---> System.Net.WebException: The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel. ---> System.Security.Authentication.AuthenticationException: The remote certificate is invalid according to the validation procedure.
at System.Net.Security.SslState.StartSendAuthResetSignal(ProtocolToken message, AsyncProtocolRequest asyncRequest, Exception exception)
at System.Net.Security.SslState.StartSendBlob(Byte[] incoming, Int32 count, 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.StartSendBlob(Byte[] incoming, Int32 count, 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.StartSendBlob(Byte[] incoming, Int32 count, 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.StartSendBlob(Byte[] incoming, Int32 count, 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.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
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)
--- End of inner exception stack trace ---
at System.Net.HttpWebRequest.GetResponse()
at System.ServiceModel.Channels.HttpChannelFactory`1.HttpRequestChannel.HttpChannelRequest.WaitForReply(TimeSpan timeout)
--- End of inner exception stack trace ---

Server stack trace: 
at System.ServiceModel.Channels.HttpChannelUtilities.ProcessGetResponseWebException(WebException webException, HttpWebRequest request, HttpAbortReason abortReason)
at System.ServiceModel.Channels.HttpChannelFactory`1.HttpRequestChannel.HttpChannelRequest.WaitForReply(TimeSpan timeout)
at System.ServiceModel.Channels.RequestChannel.Request(Message message, 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 Microsoft.SystemCenter.Cloud.SharedLibrary.Discovery.IDiscovery.GetEndpoints(String subscriptionId)
at Microsoft.SystemCenter.Cloud.SharedLibrary.Discovery.DiscoveryHelper.<>c__DisplayClass1.<DiscoverAcsEndpoint>b__0(IDiscovery service)
at Microsoft.SystemCenter.Cloud.SharedLibrary.RestCallHelper.ExecuteRestCall[TContract](Uri endpointUri, WebProxy webProxy, String accessToken, RestMethod`1 method)
at Microsoft.SystemCenter.Cloud.SharedLibrary.Discovery.DiscoveryHelper.DiscoverAcsEndpoint(String subscriptionId, Uri outsideInServiceBaseUri, WebProxy proxy)
at Microsoft.SystemCenter.Cloud.OutsideInUnitModule.DiscoveryWriteActionModule.Execute()'

Any clue?

Regards,
Khaled A. Hamad