Issue: XenForo search results in error

Please be more specific regarding the parameters of the search. What options were selected in the search form? Was this a search against all content or only wiki articles?

It was a search on the on page search box (XF default) I searched for 'häufig' in titles only, as I want to know if this lead to the wiki article with this word in the title.
See: http://awesomescreenshot.com/0e661akk2d
On your XF demo there are no results.
On my dev site, it states 'The following members could not be found: häufig.'

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.

II see the issue. Its because the word has a ä in it. Somehow that will not work in wiki search. Xenforo does not have this term in its ignore list.
I can find the same article if I search for other words in the title.

I created a "search häufig test" on the demo, and searched for the exact term "häufig" in titles only. I get the expected results. Please try searching for a non-wiki thread with "häufig" in the title.

If you are using XenForo's ElasticSearch, please try:
1. Disable ElasticSearch
2. Rebuild the wiki search index
3. Try the search again.

If the search succeeds without ElasticSearch, then there is either
1. A bug in XenForo's ElasticSearch add-on
2. A problem in your ElasticSearch configuration
3. A bug in ElasticSearch itself

We don't use ElasticSearch on our demo, so that is a possibility.