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.
Seems to happen more often after a restart, after the first 404s it will eventually start pulling posts properly then at some point the connection will go stale and it'll 404 again. It's pretty consistent for me. About to try it out on the latest pull. Nothing in the logs/dev mode. Just 404s in the console.