As stated in the title as well as in the official TYPO3 documentation the file ext_typoscript_setup.typoscript has no effect when site sets are used. Therefore the "Impersonate User" Feature of the backend module does not work.
That file should be migrated and importet correctly to work with site sets too.
Regardless of that problem, it would be nice if femanager would be providing it's own site sets.