Sounds like you might have deleted this Data Connection. There are a bunch of connections that are loaded with the system that start with qs_ that you need to keep.
I don't know of any easy way to get this connection back. But you can try to add it again from within an app in the Data Load Editor.
You would want to add a new connection on the right side. It will be a Qlik REST Connector. There are a bunch of options to fill in. Below is the string on my connector. You might be able to see the choices in mine and mimic these in your setup. Make sure you give the connector the same name as the original when you save it.
CUSTOM CONNECT TO "provider=QvRestConnector.exe;url=https://localhost/qrs/license/useraccesstype/full;timeout=180;method=GET;autoDetectResponseType=true;keyGenerationStrategy=0;useWindowsAuthentication=true;forceAuthenticationType=false;useCertificate=No;certificateStoreLocation=CurrentUser;certificateStoreName=My;queryParameters=xrfkey%20000000000000000;queryHeaders=X-Qlik-XrfKey%20000000000000000%1User-Agent%2Windows;PaginationType=None;IsOffsetStartFieldHeader=false;IsOffsetCountFieldHeader=false;IsOffsetTotalPathHeader=false;IsNextTokenFieldNameHeader=false;IsNextTokenFieldPathHeader=false;IsNextUrlFieldPathHeader=false;"
Maybe someone knows a better way.