Whenever a workflow fails to import CRM 4.0 will give you a friendly error message with the GUID of the workflow that failed. Come on! don't tell me you don't know exactly what workflow failed just by looking at the GUID? :/
Indeed, the above is not particularly helpful. However, you don’t have to go to the database to find out which workflow failed or anything like that, a simple text search for the GUID on the customizations.xml file that you are trying to import will lead you to the offending workflow.