error occurred while updating the object context

If you simply did the following this wouldn't happen:

  context.Users.AddObject(user);
  content.SaveChanges();

I suspect the problem is occurring because EF doesn't know about the AuthenticationToken object, it's not being attached to the context because it's added to a disconnected entity which is then attached to the context.

You either need to let EF handle the whole object graph connectivity situation or you need to do it all yourself. Mixing and matching like this doesn't work.