In Game #2663711 on frankfurt, my opponent immediately lost his connection, and had the jittering connection issue where it says they reconnect and disconnect repeatedly, and eventually did not reconnect to the game.
But his username changed into UNKNOWN_23382 after I reloaded the game.
It helps to go back to the log in screen and click kick and resignnat the bottom. The same thing happened to me, and I was caught in a feedback loop even after restarting my machine, but kick and cancel works.