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.
In v.0.7.1 & v.0.7.2 correctly count posts in old structure of sub category.
But if create new category & create post in sub category - counter didn't "see" posts in subcategory & didn't preview posts in main page.
Bug report in git - https://github.com/NodeBB/NodeBB/issues/3368