Ok, I think something bigger is happening.
I reinstalled 1.5.44 and still happened.
I reinstalled 1.5.43 (which I know was working 100%) and it still hangs on a win.
I started a new quest, too. Same thing...
EDIT - Ok, then decided to delete the deck folder and the card images folders.
It finally worked. Not sure what the exact problem is - either old deck lists don't work on the new build?
EDIT2 - old deck lists are fine. I put them back in the folder and they loaded up fine and played a quest match fine.
Sorry, can't narrow this down further. But at least it's up and running now.