根据说明文件：

500-599范围内的错误（如500或503）表示在尝试处理请求或服务器暂时不可用时，FCM连接服务器出现内部错误（例如，由于超时）。发送方必须稍后重试，以履行响应中包含的任何Retry-After标头。应用服务器必须执行指数退避。

Errors in the 500-599 range (such as 500 or 503) indicate that there was an internal error in the FCM connection server while trying to process the request, or that the server is temporarily unavailable (for example, because of timeouts). Sender must retry later, honoring any Retry-After header included in the response. Application servers must implement exponential back-off.

源： https://firebase.google.com/docs/cloud-messaging/http-server-ref#interpret-downstream

但是，请确保JSON请求格式正确。尝试使用 json.toString（）更新您的问题，并使用以下描述的过程检查您的API密钥是否仍然有效：检查服务器密钥的有效性

However, make sure that the JSON request is properly formatted. Try to update your question with the json.toString() and check that your API key is still valid using the procedure described in: Checking the validity of a server key