Helldivers 2 developer Arrowhead Game Studios has said it will be releasing updates on PS5 and PC this week that aim to tackle the "most serious problems" the game is currently facing, including login, matchmaking, and server load. The first of these fixes will arrive tomorrow, February 20, but Arrowhead wants us to remember that "no single update will solve all the issues."
In the config file under appdata, edit fullscreen flag to false. Its one of the first lines in the file.