InvalidOperationException "deadlock" for C# WCF SOAP service -- ConcurrencyMode in Callback fix not working

I have a WCF SOAP service in C#, on .NET 4.0. It's a vendor solution and not my own design. There are two applications at play here. Application A is the primary application. It talks to the server. Application B is a client to the primary application. Almost like a plug-in. They talk to each other over 127.0.0.1, on the local machine. A is a blackbox and I have no control over it and cannot view its source code. However, I can customize B and modify source code.

InvalidOperationException is thrown. User clicks on a button in a form on application A. Button fires an event. The event is sent to application B. I perform a SOAP service call, from B, causing the aforementioned exception:

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I tried adding CallbackBehaviorAttribute of Reentrant and then Multiple. Both failed to resolve the exception. I even set UseSynchronizationContext to false and that also did not help.

I know the problem is not the SOAP call itself. The service executes successfully, if it's initiated directly by B. The event firing on A and sent to B is the monkey wrench.

[System.Diagnostics.DebuggerStepThroughAttribute()]
[System.CodeDom.Compiler.GeneratedCodeAttribute("System.ServiceModel", "4.0.0.0")]
[System.ServiceModel.CallbackBehavior(ConcurrencyMode =System.ServiceModel.ConcurrencyMode.Multiple, UseSynchronizationContext = false)]
public partial class UADAPIClientServiceClient : System.ServiceModel.DuplexClientBase<UADClient.UADWS.UADAPIClientService>, UADClient.UADWS.UADAPIClientService {

    public object getInfo(UADClient.UADWS.UDAgent agent, UADClient.UADWS.UDInfo info) {
        return base.Channel.getInfo(agent, info);
    }
}

When I step through the code, I see that it eventually reaches a file called realproxy.cs, but it cannot find the source for it. Is that a Microsoft file? If the problem is I need to specify the CallbackbehaviorAttribute in the realproxy.cs file, I'm not sure how to, since I can't even find the file.

Any help would be most appreciated with this. Thank you in advance.