Codeigniter doesn't handle Umlaut-URLs propper

I got a Umlaut-Domain (www.homöop[...].de) for a Project, on which I'm using Codeigniter. The Problem is, that it seems like codeigniter can't handle Umlaut-Domains. When I'm using redirect() (from URL-Helper), I become redirected to an URL where Codeigniter replaced 'ö' with %26/#246;. So the whole URL looks like http://www.hom%26/#246;op[...].de/index.php/myfunction
The reason for the replace with %26/#246; is, that i set the config.php base_url to:

PHP Code:

$config['base_url']    = 'http://www.homöop[...].de/'; 


I already tried to add a Convertion-Rule for ö as a malicious character in system/core/URI.php. It solved some problems with Umlaut-URLs but it didn't solve the problem explained obove.

Do you have ideas how to fix it? Thanks!