Durham has to make two offers to every student, one is the offer of a place on the course and the second is the accommodation offer.
Your application goes first to the course admissions tutors. If they want to make you an offer, then your application is passed to the college of your choice. If they want to accept you and their subject quota has not been filled, then you will receive an offer reasonably quickly ( in Durham terms !)
On the other hand , your chosen college may, for whatever reason, decline you as a student and you application is then passed on to other colleges for consideration, until a college place is offered.
You will not receive a firm offer until Durham can offer both a place on the course and a college place.
If the AES tracker changes to say thay your application has been passed to a college, then it means that the course admission tutor wants to make you an offer, and he now has to wait for a confirmation back from college.
I have never heard of an instance where a student has been rejected because no college has a place.
So.... unless there is an error on AES, you should receive an offer, but it may take a few weeks.