You can also escape the Djinn bug in the Red Arena by changing Troops.json file. I didn't want to investigate why the message doesn't go away, but instead I decided to change the battle itself by eliminating the Djinn altogether from the battle and then changing the "invincible" condition that the Urchin has to something else (I chose poison, just because).
Replace the lines for id:1056 and id:1066 with these, respectively.
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