Broken Content Backend Module after upgrading to Neos CMS 7

I just updated a Neos project form 5.3 to 7.0 as described on Upgrade Instructions 5.3 → 7.0 - Neos Upgrade Instructions - References - Neos CMS - Neos Docs. The website is shown correctly but the Neos editor interface for the content does not work correctly:

This is shown after clicking the on burger menu icon:

This exception is thrown after clicking on the name of the site (“”) below the “Content” menu entry: