Hello!
I found a solution, to fix the Alt+Tab issue.
Solution:
Turn off fullscreen mode, you have to turn off fullscreen mode in SinglePlayer (Dont worry, you will play with the game with fullscreen borderless mode.)
Download the borderless gaming application. You can download it here: https://github.com/Codeusa/Borderless-Gaming/releases
Start the game in MultiPlayer.
Open borderless gaming.
Move Euro Truck Simulator 2 MultiPlayer to favourites.
If you want to do this with SinglePlayer, move Euro Truck Simulator 2 to favourites.
If your game is seems like a fullscreen game, you are done! Try to alt-tab out from the game, if you did everything great, you will cannot crash your game, with alt-tab.