Sadly, that didn't change anything. It still throws the same error (just with different line numbers, due to the few that were added).
I can only get the game to boot up by removing the "main_menu" AND "game_menu" part from RESTORE_FOCUS_SCREENS.
It also works if I remove [layer "game_menu"] from my "main_menu" screen and do:
define pad_config.RESTORE_FOCUS_SCREENS = [ ("main_menu", "game_menu"), "controller_remap" ]
So it seems like it doesn't really care what I put in the tuple?