The 'Wiki Articles' area has a forum header, it is not actually stored there. The 'Wiki Articles' area, actually returns no permission. You may want to review permissions. It looks like you gave Administrators custom permissions for the 'Wiki Articles' area back in the day: /admin.php?wiki/&panel=permission&groupid=3&do=customize
However, it is a bug that the header displays if the user doesn't have permission to view the current page content. You would expect the header to be considered part of the page the user can't view.
You have 2 nodes named 'Wiki Articles'. This might be an error, but this is very confusing for VaultWiki. You should rename one of them and see if they are duplicates or not: /admin.php?wiki/&panel=conflict&prefixid=0&key=wiki-articles&do=edit
'Häufig-gestellte-Fragen' was not working because of the 'ä' in its name. It seems that the route DM has been encoding its incoming titles when the title is already encoded by the caller. This is fixed for you and in the next release, but will not fix any existing articles with issues. We may have to add a tool to rebuild URLs if people encounter issues, or we may have to automatically rebuild them during the upgrade. I haven't decided yet.