This is in the known issues list at the top of this forum:
When that happens, you can force-quit the game (Alt+F4 in Windows) and restart it to return to the game in progress. There is a small chance that it won’t work (reconnecting for single-player adventures is a new feature), but better than conceding your run. Since you tried to concede though, the server might have received that request even though your client was otherwise not responding. Try the force-quit first next time, and fingers crossed that they fix the bug soon!