Does this occur for all terms? If it is just "häufig", perhaps there is something special about that term. Perhaps it is a common word and XenForo is returning the wrong error message for "The following terms were too common: häufig"
Have you tried this with VaultWiki as the only add-on? Have you tried this search while VaultWiki is disabled (assuming you have other content with that kind of title)?
Generally if you cannot reproduce a bug on our demo then it is either something specific to your site (like another add-on) or there are other factors involved that you are not currently aware of, like a specific XenForo setting, that are needed to reproduce the issue.
When I search for the term "Fake" (a known term in our demo wiki) in titles only, whether by XenForo's default "Search Everything" tab, or just on "Search Wiki Pages", I get the expected results. It does not start looking for users named "Fake"
If you cannot figure it out, you are welcome to open a Support Ticket so we can look at your dev site.