sorry for bothering 😉
I thought HTTPFS is also started automatically while restarting all the services via Ambari.
I manually started this service and configured in hue.ini the current active namenode as defaultFS and now the filebrowser is working.
There is just one open question: if a namenode takeover occurs the filebrowser would stop working since the configured server isn’t the active one. Do I have to reconfigure hue.ini anytime the active namenode switches over ?!?!