I found a solution that seems to fix it! Apparently, the game now uses Direct X 11 by default, and Umod uses Direct X 9. So you have to force HS to use DX9 again. In order to achieve that,
1. Go to your Blizzard App (Former Battlenet App)
2. Click the logo in the Top Left
3. Go to "Game Settings"
4. For Hearthstone, enable "Additional command line arguments"
5. Enter "-force-d3d9" (Without the ")
6. Click "Done"
7. Start Hearthstone and hope it works!