"400 call back error" is returned when the Callback parameter is used.
The preceding issue is caused by one of the following reasons:
The response that is returned by the application server does not contain the Content-Length header.
The Callback or Callback-var value that is passed in is invalid.
The response that is returned by the application server to Object Storage Service (OSS) must contain the Content-Length header. The size of the response body cannot exceed 1 MB. Check the Callback and Callback-var values that are passed in the Content-Length header. For more information, see Callback.