I have exactly the same problem and the same answer from the dev team (
https://forums.taleworlds.com/index.php?threads/crash-when-starting-campaign-or-sandbox.443529/). As a workaround to get at least Singleplayer running:
Go to ...Steam\steamapps\common\Mount & Blade II Bannerlord\bin\Win64_Shipping_Client
You'll find 3 exe-files there, the Bannerlord.Native.exe starts the singleplayer directly and that works for me as a workaround solution.
It clearly, atleast in my case, is not the game itself, it is the launcher version (that is the regular Bannerlord.exe), that somehow crashes on this.
Bannerlord_BE.exe is the BattleEye-Launcher, that also somehow starts up the Singleplayer Side of the game, I have no idea, how to access the Multiplayer via this method, sorry for that.
