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.
Then navigate to http://your_domain_or_ip.com:8081 and do what you need to do. You can also use redis-cli but if you're unfamiliar with Redis, like me, this is the way to go. Just don't delete anything right away. Make a backup first.