Installation of extensions from files not working

I have a stand-alone computer with no internet connection, which I want to add extensions to. The downloaded extension files are installed but the functionality in Slicer is missing.
Also when I click “more…” in the extension manager i get the following error:
“Extensions can not be installed.
Failed to load extension page using this URL: @KitwareMedical/slicer-extensions-webapp
Check that 3D Slicer is properly installed. Read more…”

This happens with several extensions such as “SegmentEditorExtraEffects” “AirwaySegmentation” and " “MarkupsToModel”

A test on another computer had the same results, but when established an internet connection an extra download was done. Still extensions do not work.
Thank you