There are a lot of menus that are hard locked where I have to quit the game such as viewing the temporary card packs.
There also doesn’t seem to be a way to resign from matches even though I run into opponents who seem to be able to do so.
You can resign if you click on the little grey button on the bottom right of the screen. The top button in the menu concedes the game.
Have you tried the “Scan & Repair” option? In the BattleNet client there is a cogwheel symbol next to the blue play button. There you will find that option.
some menus don’t have a visible back or close button , but left clicking outside of them make them close.