May be these are the issues:
Check & provide, Qlik Sense service user role as RootAdmin.
Quite often the problem comes from a miss configuration with the Qlik Repository Service (QRS) using the REST connector. If so, try to follow these steps:
1) Login in the qmc with rootAdmin grants
2) Got to the Data Connections TAB (third from the top on the left side panel)
3) Find out and modify these connections (qrs_app, qrs_appobject, qrs_event, qrs_licenseSummary, qrs_loginAccess, qrs_task, qrs_user, qrs_userAccess). For each of those change the owner to your user
4) Login in the hub, go to the Data Load Editor, now you can see the previous Data Connections in all you apps (in your working area) even from a new app
5) For each one click on the pencil icon and flag "Skip server certificate verification" notice, the data connection name change appending your user to the data connection original name
6) Make a Connection TEST, now the connection test should work
7) Come back to qmc, for each data connection rename it (removing the user name appended) and replace the original owner (INTERNAL\sa_repository)
Now the task should works.
Also check the below link.
https://help.qlik.com/en-US/sense/June2018/Subsystems/Monitoring/Content/Troubleshooting/troubleshoo...
