In the iTunes Mini-Player it is not possible to narrow down genres which contain umlauts.
If there is a genre named e.g. "Hörbuch" the genre is listed when entering "Genre/H". But entering the next letter ("ö") leading to "Genre/Hö" narrows down to a false subset not containing the desired genre any more.
It is possible though to select the genre "Hörbuch" from the "Genre/H" list. From there everything works as expected.
As far as I can tell, only "Genre" has a problem with umlauts. All other categories seem to work as expected.
Unfortunately I can't attach the screen-shots.