Build freezes on window focus / click, game logic continues (Not Responding)

[safe kite]

The problem:
So when i build and run the game, i can play it, but when I minimise the game and open it back after some time, it freezes.
When i build 2 instances to test multiplayer it either does the same or one of the builds freezes instantly.
Moreover, a ship, that is supposed to move in my game, continues to move. But the game window still is "Not Responding", and turns white. (See screenshot) So it is probably not a deadlock.

(I have no problems with editor)

Info:
I use Unity 6000.2.12f1, on windows 10, with URP. Mono build.

What i have already tried:
I turned off all networking logic and networkManager.
I turned off my suspicious OceanManager script.
I tried to debug with Visual Studio but it doesnt help much. I dont have any suspicious code lines to check.
I installed an in-game console but there are no any error logs before the freeze.

Code:
https://github.com/AmikMain/GLOOMY-WATERS/tree/amik (amik branch)

[stiff plume]

Have you checked that your Player settings are correct? I'm installing 6000.2 to see now but in earlier Unity there are two important options.

[safe kite]

Thank you for replying.
These settings look like this for me. yea i didn't check "run in background", but i never have in any other games. this should work anyways