The root site can be identified by running this in the console (F12 > Console) in the site where you have the DFFS enabled for:
_spPageContextInfo.siteServerRelativeUrl
It’s not uncommon to load jQuery in the masterpage or by another third party code, and in this scenario DFFS might detect that jQuery is not present (because it hasn’t loaded yet) and start loading it. Then the master page jQuery version starts loading and effectively “kills” the version DFFS are loading – causing this kind of error.
If putting in the link to jQuery in the staging environment works, you might want to do this for the live environment also.
Alexander