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. thanks!

@spicewiesel this is most likely a plugin conflict. Try resetting your plugins and activating them one by one.

thanks, I will reset all plugins for the next update to test which one causes the trouble.