From Ingomar Otter:

If I enter an umlaut character in Quick Search, like "ä" I get "ä" in the resulting search result page (Confluence).
I also get zero hits.

Eg. in Confluence we go a user called "Martin Küpperbusch". I don't get a hit when I search for "küpperbusch" - but I get one when searching für "K?pperbusch".
So search of non-ascii characters seems to be messed up.