400: Bad Request
Likely cause
The origin/application rejected this request as malformed.
Check
Check your application's access/error logs for the exact request that returned 400, including its headers and cookies — an oversized cookie or header set is a common cause.
Fix
Correct or reduce the offending request data (e.g. clear oversized cookies, fix malformed query parameters or request bodies) at the source generating the request.
Verify
Retry the same request and confirm it now returns a normal response.