Fail to subscribe platform event of a sandbox using workbench

Question

I failed to subscribe platform event of a sandbox using workbench. The error code is "403::Unknown client". "streaming api developer guide" says the error code means The server deleted the client CometD session due to a timeout, which can be caused by a network failure. The client must perform a new handshake and reconnect.

However, I can subscribe platform event of a product or developer org, under the same network, with the same browser environment.

I suppose sandbox org can't support platform event. Is it a known issue or how should I troubleshoot this?

Unknown exception occurred. Removing offending listener or subscription.
[
  {
    "advice": {
      "interval": 0,
      "reconnect": "handshake"
    },
    "channel": "/meta/subscribe",
    "id": "8",
    "error": "403::Unknown client",
    "successful": false
  },
  "Invalid argument: expected subscription, not undefined"
]

8. Subscription Failure: 403::Unknown client
{
  "advice": {
    "interval": 0,
    "reconnect": "handshake"
  },
  "channel": "/meta/subscribe",
  "id": "8",
  "error": "403::Unknown client",
  "successful": false
}