So there is the next problem - to me that´s totally weird:
I installed a plugin called "Intense" from codecanyon. This plugin comes with custom post types such as portfolio, news etc. Now here is what´s happening:
I created posts of the new post types and I also created a page where those posts are automatically shown. If I save such a custom post as draft and click view afterwards, I can see a normal blogpost with my content.
But after publishing it, I get "Page not found" when klicking on "view" (bringing it back to "draft" brings the "finding" back also)
However, those custom posts are sill shown on the page where their feed is shown - but you can´t click on them unless you want to get the "page no found"-message.
And now for the weirdest part: If I create a PAGE with the exact same title as the custom post type, the links on my page, where the feed of the custom post type is shown, suddenly lead to the newly made page...
I am totally confused now...