Thanks @MJ, Thanks @PitaJ
both hints helped. My nodebb did not load pages like themes/skins properly, so I tried to get websockets to work. I bypassed my shared hoster env by using an self compiled nginx on a non-privileged port and ssh tunnel (to prevent firefall changes) on that system, and that way I got websockets. Now I was able to load the theme page and so I switched to another theme, restarted and switched back to my current theme. Afterwards I stopped my temporary nginx-ws-tunnel-solution and the mobile page works for me now.
Next step: Find a solution for a permanently working websocket setup on my hosters systems.
I just tested to create a new group and following two issues occured
bug: using svg-images for header works fine until "save" gets pressed. save responds with "error".
feature-request: I'd like to set the background-size to contain and center the position.