Your time on the screenshot says 11.03. I assume this happened when trying to enter battle in an event that expired at 11.00. If you were already in battle at 11.00, you can complete the battle. If afterwards you attempt to enter battle again - f.ex. on a different tier - you will see the error message.
You can se in the picture all enemies dead except 1... 🤷🏻♂️ I think is a bug or an error. But critical closing is more relevant, this message only appears one time
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Your time on the screenshot says 11.03. I assume this happened when trying to enter battle in an event that expired at 11.00. If you were already in battle at 11.00, you can complete the battle. If afterwards you attempt to enter battle again - f.ex. on a different tier - you will see the error message.