Okay, I've found out the source of it.
It's because you've used special BattleUI frame as custom layout frame for loading screen.. BattleUI is actually root frame for Battle.net UI. Apparently trying to use it for loading screen is pretty disruptive :D And something from SC2 cannot recover without a full game restart.
By curiosity I've tried to use couple of other frames that should not be used for loading screen, but none of it resulted in broken game.
I don't know what made you use custom layout, and why did you actually pick BattleUI. But this together was worst case possible, haha.
Generally editor should report about issues like that, but I haven't been seeing anything in the log. So it took a while to find.
In fact it can be considered as bug, because ideally maps should be sandboxed in SC2. That is nothing you do inside the map should affect the game itself after you quit the map. And destroying Battle.net UI is certainly something that negatively affects user experience ;D
To summarize, to fix it just just go to *Map -> Map Loading Screen*. Unlink *BattleUI* frame. OK.
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Oh my God, you found the source of the bug. I almost can't believe it :D
THANK YOU!!! If you happen to visit Warsaw or if we meet anywhere else by chance you're getting a vodka (or whatever else you drink) from me xD
How the hell did you figure out that it was the frame? Did you like run the map just in code, matrix style, or something?
Honestly, I have no idea why I had that frame up. I must have added it while adding other loading screen stuff cause it sounded cool or something :D
"BattleUI? Like Battle.net? Gee, can't go wrong with that, right?" xD Fuuuuuuuu... Those kinds of bugs are the worst.Tiny stuff that you think only adds a little particle there, a semi-colon there. Ends up crashing your stuff.
As for the Allied Commanders. Good thing it wasn't that cause I am actually using some stuff from there :D
Damn, what else can I say. I'll keep testing it out properly now. You saved my life xD Thanks again.