Error in Asynchronous Plugin

We're getting a very strange error on some servers on a customer. In short, when we distribute a Campaign Activity we have a plugin registered against the Post Update of the Campaign Activity. This does some stuff as soon as the Actual Start date is updated.

The problem is this, we're getting an error almost immediately in the Async Operations that looks like it is hitting an error before our plugin code executes... this is the error:

Unhandled Exception: System.ServiceModel.FaultException`1[[Microsoft.Xrm.Sdk.OrganizationServiceFault, Microsoft.Xrm.Sdk, Version=5.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35]]: Invalid data
Detail:   <OrganizationServiceFault xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.microsoft.com/xrm/2011/Contracts">    <ErrorCode>-2147220989</ErrorCode>    <ErrorDetails xmlns:d2p1="http://schemas.datacontract.org/2004/07/System.Collections.Generic" />    <Message>Invalid data</Message>    <Timestamp>2013-01-24T14:46:40.9336859Z</Timestamp>    <InnerFault>      <ErrorCode>-2147220970</ErrorCode>      <ErrorDetails xmlns:d3p1="http://schemas.datacontract.org/2004/07/System.Collections.Generic" />      <Message>System.ArgumentException: Invalid data</Message>      <Timestamp>2013-01-24T14:46:40.9336859Z</Timestamp>      <InnerFault i:nil="true" />      <TraceText i:nil="true" />    </InnerFault>    <TraceText i:nil="true" />  </OrganizationServiceFault>    

So, we're getting no trace text whatsoever, and no stack trace either. The very first few lines of our execute looks like this:

public void Execute(IServiceProvider serviceProvider)
        {
            if (serviceProvider == null)
            {
                throw new ArgumentNullException("serviceProvider");
            }

            // Construct the Local plug-in context.
            LocalPluginContext localcontext = new LocalPluginContext(serviceProvider);

            localcontext.Trace(string.Format(CultureInfo.InvariantCulture, "Entered {0}.Execute()", this.ChildClassName));

So we're not getting to that trace. 

We're on CRM2011 - UR 11 if that makes a difference.

Any help much appreciated.

Thanks

Conor.