I've see this happen several times now. I'm in a game, and a reload the page. Instead of getting back to the game, the game loads and I'm taken to the main screen, not back into the game I was in.
At 7PM UTC each day happens a server reset/switch. That was introduced some time ago to fight server overload. If you play a game with no page reloads necessary, you only realize that after the game is over, when it will not move you back to the main screen.
However, if you do a page reload after the switch in a game that was started before, you will be taken to the main screen, as the new server doesn't know about your ongoing game.
Obviously this is not a good situation to be in for any players playing during that time. Work is being done to change how the game communicates internally, but that takes time. A discussion has also be brought up to change the time of the switch to make it affect hopefully less players.