Watcher Action Webhook (Google Meet space) errored

I am trying to create a Watcher monitor to check the number of documents of a query and send a message to a Google Webhook when the condition is met. The query and the condition work right. However when the condition is fulfilled, the triggered action fails with a 400 Error and a message: "Your client has issued a malformed or illegal request. ". The details of the webhook action configuration are shown below:

"webhook": {
        "scheme": "https",
        "host": "chat.googleapis.com",
        "port": 443,
        "method": "post",
        "path": "/v1/spaces/--------/messages",
        "params": {
          "token": "------",
          "key": "------"
        },
        "headers": {
          "Content-Type": "application/json"
        },
        "body": "There are {{ctx.payload.hits.total}} documents in your index. Threshold is 10."
      }

The original url of the webhook is as following: https://chat.googleapis.com/v1/spaces/------/messages?key=-------&token=--------

I have no idea why this error is spammed. Any help?