I still think you have some strange apache2/nginx redirecting config in place. the url that the nagvis module is trying to reach is:
/nagvis/frontend/nagvis-js/index.php?mod=Map&act=view&show=demo-overview&header_menu=0
and somehow this redirects to your icingaweb2 instance which causes this issue.
http(s):///nagvis/frontend/nagvis-js/… should be provided by the nagvis installation and not be redirected to icingaweb2.