"unknown error" when starting a session from my account linked to Google Play

@gaijin_JDM_8 Try starting the game as a new player. Play through your device without logging in with Facebook or Google. Then, when you finish the tutorial, go to in-game settings (the gear icon) and go to accounts. Tap LINK to whichever account you prefer and most possibly you will manage to connect. It worked for me, as I had the same problem for days.