We had to create a spectific “technical” user in our Active Directory but it was not available in WorkflowGen after synchronization (but other users were correctly synchronized).
Cause of the error in our case : First Name and Last Name were not filled in the AD.
Solution : There are certainly mandatory fields needed by workflogen to work correctly. Username, FirstName and LastName seem to be part of them. You need to complete theses fields in your AD to enable the synchronization of the user.