Here is an update to this long and curious story. The CableToGo HDMI Adapter only worked once on the machine. However, I tested the adapter on two other laptops and it worked without a hitch. Assuming that it was driver related, we did a full reset on the machine without success. An engineer came today with a new motherboard and an adapter. The new adapter worked.
Conclusion: My Dell XPS 12 did not like the CableToGo displayport-HDMI adapter. It is hard to say if the adapter was faulty (It worked on other laptops) or if it is a general incompatibility problem. The fact that it worked once after reinstalling the driver was just serious bad luck.