You should stick to only US-ASCII characters for your URL scheme, otherwise you will start encountering problems. Only ASCII is permitted in current implementations. In the future, IRIs will replace URIs, but that time is a long way off yet.
I'll replace "überwachung.html" to "uberwachung.html"
Even better, replace it with "ueberwachung.html", as that's the accepted way in German of spelling when umlauts aren't available.
I'm not quite sure of the technicalities, but in "real life" UTF-8 encoded umlauts don't seem to be a problem, at least if the server uses UTF-8 as its encoding, see for example the German wikipedia, e.g. [de.wikipedia.org...] or [de.wikipedia.org...]