Make sure you only have 1 browser open on the game client page. If you have 2 browsers open on the game client page you will get that error. And don’t click return to game from the WGT main page. Doing that will open a 2nd browser on the game client page causing that error.
Open a single browser directly to the game client page and it should allow you to continue your game. The best thing to do is to set your browser start up page to the game client so when you start your browser it loads straight to the game client page. I have all the browsers that I use to play WGT set up that way and never have a problem restarting a game.