@Jummy, if only one device is having issues submitting the data to the server, then it could be a configuration issue. Could you share a screenshot of the Settings>Server that is having a submission issue and a screenshot of the Settings>Server that does not have a submission issue? These screenshots should help us troubleshoot your issue.
@Jummy, could you correct the URL to https://kc.kobotoolbox.org instead of the one you are currently using, https://kc.kobotoolbox.org/, to see if this helps you solve your issue?
In addition, please also ensure that the project (for which you are trying to upload the data to the server) is there on the server.