KM treats the %C3%A4% in the URL as "the Unicode token %C3% followed by the text A4%, which is why the URL doesn't work. (See the Other Uses for Tokens section of the KM manual for more about this.)
How you handle this depends on your macro. You can turn off token processing in the "Open URL" action if the URL is static test, but that breaks things if you are passing in a variable. So, instead, try "escaping" the % characters with another % so your URL text becomes %%C3%%A4%%.
How/where you do this will also depend on your macro -- if you post it then people can offer more specific help.
My bad -- somehow an extra % snuck into your first-posted URL and I didn't pick up on it. The actual URL is https://de.wikipedia.org/wiki/Liste_der_Hauptst%C3%A4dte_der_Erde, which will work with all processing turned off or by escaping the %s as https://de.wikipedia.org/wiki/Liste_der_Hauptst%%C3%%A4dte_der_Erde.