"Payload is not correct" error while using sendActivity of Bot Framework SDK

Question

From 04:30 JST to 18:30 JST on 9/13, sending a message using Bot Framework SDK v4's sendActivity returned a "Payload is incorrect" error.

The error occurred when sending a simple text.
https://github.com/microsoft/BotBuilder-Samples

However, after 18:30 on 9/13, it started returning correctly.
Does anyone know why this is?

I am a service provider and need to explain to my clients why the service was not working.

Response Details

CompletionException: com.microsoft.bot.connector.rest, {"error":{"code": "BadSyntax", "message": "Payload is incorrect"}}

Version

Bot Framework for Java SDK 4.14.2