I have two WCF services on 2 different and physically separated IIS servers.

Both may call a function SyncExternalServer where they collect data for an update an send it to the other server.

The services are identical. Both services are set to ConcurrencyMode.Multiple

[ServiceBehavior(ConcurrencyMode = ConcurrencyMode.Multiple)]
public class Service : IService, IDisposable

Now I can see in the logs in rare situations that both servers call SyncExternalServer at the same time. In this situation they block each other. Both calls to the external WCF service are aborted after the given timeout with a System.TimeoutException.

I can see that the servers can be connected from other computers and they react and return correct result in the time of the pending call.

Here are some extracts of the logs

SVR-1
...
2022-04-06 07:21:41,303 [   59] INFO  My.Service SyncExternalServer start svc1.mysite.com -> svc2.mysite.com
2022-04-06 07:21:41,306 [   59] DEBUG My.Service SyncExternalServer StartServerSync svc1.mysite.com -> svc2.mysite.com. SyncStart=06.04.2022 05:21:41, LastSync=06.04.2022 05:21:04
2022-04-06 07:21:41,313 [   59] DEBUG My.Service SyncExternalServer ReadAllDeletedSessions svc2.mysite.com -> 
2022-04-06 07:21:41,316 [   59] DEBUG My.Service SyncExternalServer start sending data svc1.mysite.com -> svc2.mysite.com
2022-04-06 07:21:48,112 [    1] INFO  My.Service Unregister 638593D9-41DA-4573-BB81-5FE96A7270E7 -> 0
2022-04-06 07:21:51,338 [   59] ERROR My.Service SyncExternalServer call to SynchronizeServer failed svc1.mysite.com -> https://svc2.mysite.com//XYZServer/My.Service.svc Beim Anforderungskanal ist während des Wartens auf eine Antwort nach 00:00:10 eine Zeitüberschreitung aufgetreten. Erhöhen Sie den an den Aufruf übergebenen Zeitlimitwert auf "Request", oder erhöhen Sie den SendTimeout-Wert für die Bindung. Der für diesen Vorgang zugewiesene Zeitraum war möglicherweise ein Teil eines längeren Timeouts.
2022-04-06 07:21:51,340 [   59] DEBUG My.Service SyncExternalServer call to SynchronizeServer failed
System.TimeoutException
...

SVR-2
...
2022-04-06 07:21:41,050 [   11] INFO  My.Service SyncExternalServer start svc2.mysite.com -> svc1.mysite.com
2022-04-06 07:21:41,050 [   11] DEBUG My.Service SyncExternalServer StartServerSync svc2.mysite.com -> svc1.mysite.com. SyncStart=06.04.2022 05:21:41, LastSync=06.04.2022 05:17:08
2022-04-06 07:21:41,050 [   11] DEBUG My.Service SyncExternalServer ReadAllSessions svc1.mysite.com -> 
2022-04-06 07:21:41,050 [   11] DEBUG My.Service SyncExternalServer start sending data svc2.mysite.com -> svc1.mysite.com
2022-04-06 07:21:47,813 [   13] INFO  My.Service Reregister 638593D9-41DA-4573-BB81-5FE96A7270E7 F3AB411C-8DF1-462B-A829-272AF93134C7 -> 5
2022-04-06 07:21:56,070 [   11] ERROR My.Service SyncExternalServer call to SynchronizeServer failed svc2.mysite.com -> https://svc1.mysite.com/XYZServer/My.Service.svc Beim Anforderungskanal ist während eines Sendeversuch nach 00:00:15 eine Zeitüberschreitung aufgetreten. Erhöhen Sie den an den Aufruf übergebenen Zeitlimitwert auf "Request", oder erhöhen Sie den SendTimeout-Wert für die Bindung. Der für diesen Vorgang zugewiesene Zeitraum war möglicherweise ein Teil eines längeren Timeouts.
2022-04-06 07:21:56,070 [   11] DEBUG My.Service SyncExternalServer call to SynchronizeServer failed
System.TimeoutException
...

You can see that the function that performs the sync start at the same time. After collecting the data the service starts sending with "SyncExternalServer start"

In the meantime on other threads an Unregister call arrives at SVR-1 and a Reregister call arrives at SVR-2 (see different thread numbers). Both are handled and succeed including al DB jobs it has to perform. So I can see that my server is reentrant. But the the external call fails. Also I can't see that something arrives on the other server. Usually I have DEBUG output here too.

Any ideas or hint where to look for?

Is there anything else to do to make the service reentrant and concurrent?

PS: Enlarging the timeouts have no effects! As you can see the timeout on one server is 10secs on the other it is 15secs.

Here is the code, that I use to call the external service:

try
{
    // Prepare the endpoint and the binding
    var binding = new System.ServiceModel.BasicHttpBinding();
    var endpoint = new System.ServiceModel.EndpointAddress(extServer);

    if (extServer.ToLower().StartsWith("https:"))
    binding.Security.Mode = BasicHttpSecurityMode.Transport;

    // Set large buffers
    binding.MaxBufferPoolSize =
    binding.MaxReceivedMessageSize =
    binding.MaxBufferSize = 1000000;

    // Set the timeouts
    binding.OpenTimeout =
    binding.ReceiveTimeout =
    binding.SendTimeout = new TimeSpan(0, 0, iSyncSendTimeout);

    // Now send the data in chunks.
    using (MyService.ExtMyService.ServiceClient service = new MyService.ExtMyService.ServiceClient(binding, endpoint))
    { 
    ...
        bSucceeded = service.SynchronizeServer(strData);
    ...
    }
}
catch (Exception e)
{
    ...

The receiving code is just:

// Interface definition
public interface IService
{
    ...
    [OperationContract]
    bool SynchronizeServer(string data);

...

// Service implementation         
[ServiceBehavior(ConcurrencyMode = ConcurrencyMode.Multiple)]
public class Service : IService, IDisposable
{
...       
    public bool SynchronizeServer(string data)
    {
    ...