I only can acess the top level forums
So i installed and kind of configured the forum, got a page with the correct slug, and i can see the forum home page.
But when i start a new topic, it doesn't show.
Do i have to create a custom page for every single topic what the uers might add? Or do i miss something profanely easy?
wpForo creats Forum page on installation with /community/ slug. And you don't need to create custom page for each topic. I think you've done something wrong.
If you create custom forum page you should know these:
1. The initial /community/ page should be deleted permanently, not just put in "trashed".
2. New page slug should be unique, and it should be put in Dashboard > Forums > Settings > "Forum Base URL" admin page.
3. The new page should contain [wpforo] shortcode.
Also please make sure you use WordPress permalinks. wpForo doesn't work with default ?p=1 URL structure. Check it in Dashboard > Settings > Permalinks admin page.