Within the limitations of the forum rules I'm curious as to what precisely went wrong and how it was fixed.
I gather the Garmin server is sending back a bad unl file for certain models. Is that a recent thing and if not why did it work fine until a few days ago?
I can fill in some of the blanks as to one way GN might have diagnosed the problem using the GarminDevice.xml file.
The one question I'd really like to know the answer to (and which may not be answerable here) is what the fix was and how it worked. It certainly did something interesting.