MS Graph Exception regarding Certificate - asp.net-web-api

Question

We have a .net web application (outlook web addin) that is running on-premise on two WindowsServer-2012 R2. This application makes calls to the Microsoft Graph API. Since one week the esception at the an of the question occurs, but only on one server.
The application didn't change since may. We checked also all certificates in IIS, but they are all ok.
When calling the graph api the following exception occurs, as explained only on one server:
Microsoft.Graph.ServiceException: Code: generalException
Message: An error occurred sending the request.
---> System.Net.Http.HttpRequestException: An error occurred while sending the request. ---> 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.TlsStream.EndWrite(IAsyncResult asyncResult)
at System.Net.ConnectStream.WriteHeadersCallback(IAsyncResult ar)
--- End of inner exception stack trace ---
at System.Net.HttpWebRequest.EndGetResponse(IAsyncResult asyncResult)
at System.Net.Http.HttpClientHandler.GetResponseCallback(IAsyncResult ar)
--- End of inner exception stack trace ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at Microsoft.Graph.RedirectHandler.d__6.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at Microsoft.Graph.RetryHandler.d__9.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at Microsoft.Graph.CompressionHandler.d__2.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at Microsoft.Graph.AuthenticationHandler.d__16.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at Microsoft.Graph.HttpProvider.d__19.MoveNext()
--- End of inner exception stack trace ---
at Uniqa.OLAddin.EBFWeb.Helpers.GraphHelper.d__7.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
at Uniqa.OLAddin.EBFWeb.Controllers.DoWFController.d__5.MoveNext()

Answer 1

Might it be that one of the servers hasn't got the latest root CA updates?
The SSL certificate from graph.microsoft.com was issued September 15th which could coincide with your problems.
The certificate chain is this:
Try to open the website https://graph.microsoft.com/v1.0/me/ on the server in question. If you get a certificate error, then try updating root certs (it's a Windows update).