There’s several things you could try, if you haven’t already.
If you have any special input devices (controllers, joysticks, etc) plugged in, try unplugging them before starting Battle Brothers.
Run the game with admin rights (from outside Steam, but with Steam running).
Put the game on the exception list of your anti-virus solution, or disable it temporarily.
Use the ‘Verify Integrity of Game Cache’ function in Steam by right-clicking the game’s entry in the Steam library, choosing ‘Properties’ and navigating to ‘Local Files’.
Update your video drivers to the most recent version.
Let me know if the problem persists.
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