Issues with Importing organisation from CRM 4 to CRM 2011

Hi All

We are facing issues with importing a CRM 4 organisation into CRM 2011, we get the following error.

System.Exception: Action Microsoft.Crm.Tools.Admin.UpgradeWorkflowActivitiesAction failed. ---> Microsoft.Crm.CrmException: The custom activity does not contain a valid CrmWorkflowActivity attribute
at Microsoft.Crm.Tools.Admin.UpgradeWorkflowActivitiesAction.CustomActivityData.LoadActivityParameters()
at Microsoft.Crm.Tools.Admin.UpgradeWorkflowActivitiesAction.CustomActivityData.get_ActivityName()
at Microsoft.Crm.Tools.Admin.UpgradeWorkflowActivitiesAction.MarshalByRefWorkflowActivityAction.UpgradeCustomActivityBatch(Int32 batchSize, Guid organizationId)
at Microsoft.Crm.Tools.Admin.UpgradeWorkflowActivitiesAction.MarshalByRefWorkflowActivityAction.UpgradeCustomActivityBatch(Int32 batchSize, Guid organizationId)
at Microsoft.Crm.Tools.Admin.UpgradeWorkflowActivitiesAction.Upgrade(Guid organizationId)
at Microsoft.Crm.Tools.Admin.UpgradeWorkflowActivitiesAction.Do(IDictionary parameters)
at Microsoft.Crm.Setup.Common.CrmAction.ExecuteAction(CrmAction action, IDictionary parameters, Boolean undo)

Now, I have done some research and followed the instructions in 

http://blog.cajunmchicken.com/2011/04/migration-to-crm-2011-custom-activity.html

But removing these dlls doesn't resolve the problem because when I try to import the organisation again, it says that it is successful but it doesn't import any of the components, only the reports.

My question is, is there a way to find out what the custom attribute is? As the error message (stack trace) is not very helpful ie: it doesn't identify the custom activity that is not valid.

Kind Regards

Charles