I found out that by changing an Apache2 Header directive
Header set Content-Security-Policy "default-src 'self';"
it is possible, in my installation of Hubzilla on Raspberry, to bring out an Addons screen manager in the Addons - Admin menu of Hubzilla. However, the buttons of the Addons Manager do not work. Wondering if there is any hidden configuration option somewhere to do the same thing working?