For whatever reason, Mapster is not allowing the Escape button to function in any way in game. It won't bring up the menu window, can't cancel target, it just does nothing.

Edit - It isn't fully disabling Escape in-game. It will work to close a whisper window from the WIM addon. It seems to be blocking all input to the game itself.

I confirmed that it wasn't my keyboard or mouse (G600).

Disabled all addons, brought them back in chunks until Esc stopped working. Then confirmed with everything off but Mapster.

I had the same issue. I installed Leatrix Maps just to have something work while this is being worked on and I found that it was having the exact same issue, but only when the map wasn't anchored. I didn't see an option in Mapster to lock the map frame so I couldn't test to see if that would fix it. Not sure if this is helpful information, but I figured I'd add it anyway!

Same issue here

Same here as well. I did find that if I interact with an NPC or open the achievement window for example that ESC works after that for bringing up the main game menu etc as usual. It's just broken until you manage to interact with an NPC or apparently use the achievements screen. Other screens like LFG or the Social/guild screens do not allow it to work. You can ESC to get out of them, but you can't use ESC to get the main game menu up unless it was an NPC or the achievements screen.