the game doesn't actually start for me, it just does a loading circle for a split second before going back to the main menu, with no changes. I can access options and exit, but the game is unplayable
only in browser or also downloaded? It tries to connect to the cloud save (bad internet connection or vpn might interfere). You can try running it offline (turn off your internet connection) maybe it works after doing it 1x. It works for thousands of players, so I can't help you more, sorry...
