I'm experiencing the same issue, but clicking "Cancel & Resign" only flashes the player matching lobby, and then resumes the loading page.
It started for me when I mis-clicked on the wrong card for my first decision of the game, Lord Rattington and Revenge Witch took their first turns. I kept trying to enter "/undo 12", "/undo 13", "/undo 14", etc, to see how far I needed to go to get back to my turn. Eventually the game froze. When I refreshed the page, it was just an endless loading screen.
I hard-refreshed in case I needed a new client. That didn't help. Then I opened the site in Chrome's Incognito Mode and logged in, only to get the same issue.