I'm playing on Linux, but this is a common problem for Mac users too. The screen gets white after the game loaded. I've read an old discussion on the discord group saying "just use the static menu background in the additional files folder", but that's just not enough. I think I found the static menu he was talking about (in /Modules/Medieval_Conquest/"Choose UI here players"/Menus/"Main Menus"/"Static Menu with shaders"/main_menu_static.dds) but I don't know where to place it.