this issue is related to server/redis, NOT nodebb itself.
I have created dedicated Redis (3.0) cluster via Azure portal and seems to working fine from the 1hour.
I need to add rest of the plugins and check if they are not cause any issue but seems that problem was with the server/newest redis
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.