When having Active Directory Groups Based security management it is not possible to use the Workflow standard functionality : assign user to workflow element based on security role.
The error says that it is unable to load a session for the User.
However when assigning the element directly to user, if the user has insufficient rights, the workflow will throw an error saying that the rights aren't sufficient.
That means there is a logic into the workflow functionalities that is able to read rights inherited from Active Directory Groups.
Using this logic to make it possible to assign workflows elements to security roles even in an Active Directory Group based security context would be AWESOME !