Grey screen or "session expired" means that you have banned. 

Or you can get a pop-up with info about your ban.

It's one-day ban, so you can enter the game after this period.

This is caused by multi-accounting — supposedly you use more than two accounts. Sometimes it may be caused by shared internet or using VPN / other services.

Please wait one day until ban will be ended, or you can change your ip-address, and then you get an access to the game.