There's currently a full marks event on at the moment.
The error message is caused by having the "gems/coins cost 0" hack on, which doesn't interact well with events such as carnival, club constellation and in this case, full marks. Turning that off before entering the game (and only turning it on when you need to use it) should help if you see a similar error message again.
Usually it is highly recommended that mod users don't log in during full marks anyway, as you could get caught. Luckily for you it looks like you are in year 1 so the chance of being caught is low.