WebAPI Runs "connection-refused" from Android Device

I implemented a WebAPI in GAE/PY (standard environment).

I ran this WebAPI from an Android device.
connection-refused.

I have checked the following, but what do you think is possible?
Also, is there a condition for connection-refused in the official GCP document?

■ Confirmation
1. No reproducibility
　It was resolved in about an hour.It has not occurred since then.
2. There was no trouble on the GCP side
　There were no GCP problems during the time of the event.
　https://status.cloud.google.com/
3. No client restrictions
　There is no IP restriction in the firewall.
　No IAP restrictions
4. No request log in Stackdriver logging
　There was no request_log when the event occurred.