Timeout while fetching URL

What is the problem? Please be detailed.
I have an ODK aggregate and flask web application deployed in compute engine.
And I have created JSON publisher that publishes existing as well as new submissions data to one
http://my_ipaddress/test URL.(in my Compute Engine).

What ODK tool and version are you using? And on what device and operating system version?

What steps can we take to reproduce the problem?

What you have you tried to fix the problem?

Anything else we should know or have? If you have a test form or screenshots or logs, attach here.
org.opendatakit.aggregate.task.UploadSubmissionsWorkerImpl: uploadAllSubmissions: org.opendatakit.aggregate.exception.ODKExternalServiceException: java.net.SocketTimeoutException: Timeout while fetching URL: http://my_ipaddress:8080/test?

    at org.opendatakit.aggregate.externalservice.JsonServer.sendRequest (JsonServer.java:148)
    at org.opendatakit.aggregate.externalservice.JsonServer.insertData (JsonServer.java:197)
    at org.opendatakit.aggregate.externalservice.AbstractExternalService.sendSubmission (AbstractExternalService.java:138)
    at org.opendatakit.aggregate.task.UploadSubmissionsWorkerImpl.sendSubmissions (UploadSubmissionsWorkerImpl.java:322)
    at org.opendatakit.aggregate.task.UploadSubmissionsWorkerImpl.streamSubmissions (UploadSubmissionsWorkerImpl.java:304)
    at org.opendatakit.aggregate.task.UploadSubmissionsWorkerImpl.uploadAllSubmissions (UploadSubmissionsWorkerImpl.java:198)
    at org.opendatakit.aggregate.task.gae.servlet.UploadSubmissionsTaskServlet.doGet (UploadSubmissionsTaskServlet.java:109)