AppDomainUnloadedException using MAF (Microsoft Addin Framework)

Question

I'm having a AppDomainUnloadedExcpetion from time to time on some machines when calling an addin hosted using MAF (System.AddIn). This is what my pipeline looks like:

[AddInContract] 
public interface IGatewayV2 : IContract
{
     ITopUpResultContract TopUpPhoneAccount(ITopUpRequestContract request);
}

This is my Host Adapter:

[HostAdapterAttribute()]
public class GatewayContractToViewHostSideAdapter : Dogs.Pipeline.HostView.IGatewayV2
{
    private Dogs.Pipeline.Contracts.IGatewayV2 _contract;
    private ContractHandle _handle;

    public GatewayContractToViewHostSideAdapter(Dogs.Pipeline.Contracts.IGatewayV2 contract)
    {
        _contract = contract;
        _handle = new ContractHandle(contract);
    }

    public TopUpResult TopUpPhoneAccount(TopUpRequest request)
    {
        var topupRequestViewToContractHostSideAdapter = new TopUpRequestViewToContractHostSideAdapter(request);

        return
            new TopUpResultContractToViewHostSideAdapter(
                _contract.TopUpPhoneAccount(topupRequestViewToContractHostSideAdapter));

    }
}

always on the host adapter side I have a topupRequestViewToContractHostSideAdapter:

class TopUpRequestViewToContractHostSideAdapter : ContractBase, ITopUpRequestContract
{
    private TopUpRequest _topUpRequest;

    public TopUpRequestViewToContractHostSideAdapter(TopUpRequest topUpRequest)
    {
        this._topUpRequest = topUpRequest;
    }

    //properties here
)

and a TopUpResultContractToViewHostSideAdapter which handles the reference to the external appdomain:

  public class TopUpResultContractToViewHostSideAdapter : TopUpResult
  {
    private ITopUpResultContract _contract;
    private ContractHandle _handle;

    public TopUpResultContractToViewHostSideAdapter(ITopUpResultContract contract)
    {
        this._contract = contract;
        _handle = new ContractHandle(contract);
    }

    //properties here
  }

On the addin side instead I have the following code:

[AddInAdapter()]
public class GatewayViewToContractAddInSideAdapter : ContractBase, Dogs.Pipeline.Contracts.IGatewayV2
{
    private Dogs.Pipeline.AddinViewV2.IGatewayV2 _view;

    public GatewayViewToContractAddInSideAdapter(Dogs.Pipeline.AddinViewV2.IGatewayV2 view)
    {
        this._view = view;
    }

    public ITopUpResultContract TopUpPhoneAccount(ITopUpRequestContract request)
    {
        var result = _view.TopUpPhoneAccount(new TopUpRequestContractToViewAdapter(request));
        return new TopUpResultViewToContractAdapter(result);
    }
}

where the TopUpResultViewToContractAdapter is:

public class TopUpResultViewToContractAdapter : ContractBase, ITopUpResultContract
{ 
    private TopUpResult _topUpResult;

    public TopUpResultViewToContractAdapter(TopUpResult result)
    {
        this._topUpResult = result;
    }

    //properties here
}

and the TopUpRequestContractToViewAdapter is:

public class TopUpRequestContractToViewAdapter : TopUpRequest
{
    private ITopUpRequestContract _contract;
    private ContractHandle _handle;

    public TopUpRequestContractToViewAdapter(ITopUpRequestContract contract)
    {
        _contract = contract;
        _handle = new ContractHandle(contract);
    }

    //properties here
}

Everything seems compliant with what I've read so far and in particular with what I can read at this link: http://msdn.microsoft.com/en-us/library/bb384186(v=vs.100).aspx So I'm wondering if I'm doing something wrong or there is a possibility that the pipeline doesn't work on certain machines (as I'm having problem not on all the machines I'm using for testing).

Thanks for any hint.