I battled today (13.07.2019) the balance gym 3 times. When i was about to kill the last Pokemon of the Leaders the Battle ended and chat said:
"A battle error has occurred! Check the console for details and report this bug to the Pixelmon developers"
Also now after I challanged already 3 times, the chat say that u can only battle this trainer once a minecraft day, althought i battled it already more times xD