When I looked at your site, the "Handbuch" tab is not selected when you visit the wiki. This would make it impossible to view the wiki navigation, including "Post New Article" and "Upload New File". Are you using a mod to manage the navbar tabs on your site? If so, you may have configured the mod incorrectly when you created the Handbuch tab. Otherwise, if you are using vBulletin's default solution, please open the Navigation Manager, check that a "Wiki" or "Handbuch" tab exists, and that it contains links such as "Post New Article".
Unfortunately, 3rd party navigation managers are out of the scope of VaultWiki. They require manual input of navigation entries. You can always compare your entries against the vBulletin Navigation Manager entries to make sure you have them all there, but you still need to make sure the entry is working properly and gets selected when you actually visit the wiki.
As for creating new wiki pages from the forum view, if you have permission to Post Threads in that forum (maybe there are custom permissions for that forum), and check the forum settings to make sure the forum is not closed.