Last night, I was looking through my collection as I waited to get paired into the next round of a league, thinking about changes I could make to my deck at the end. I opened the "Quantity" box to change the restriction on my card search just as I was getting paired into my match. The result was...
I had this issue today! Following steps from this link fixed it for me (particularly the last section, where you delete the 2.0 folder from %LocalAppData%\Apps. If you try to do this and it says you have files from it open, open your task manager and End Task on anything titled MainNavigation...