Investigation with a user encountering this issue revealed that it was linked to obsolete session cookies on the client (browser) side.
In case you run into this issue again, or if you get in a game as a spectator while you are one of the players, it means that something is wrong with your user session.
The solution is: use the back button to get back to the mainsite; log out; log in; then browse back to your game.
In case you run into this issue again, or if you get in a game as a spectator while you are one of the players, it means that something is wrong with your user session.
The solution is: use the back button to get back to the mainsite; log out; log in; then browse back to your game.