When I acccess the URL http://efg-zoar.de/ in Firefox with the HttpFox extension installed, it shows this response header.
(Status-Line) HTTP/1.1 301 Moved Permanently
Date Thu, 02 May 2013 21:45:41 GMT
This shows that you have Apache installed, and something is performing a server-side redirect. As an earlier post mentioned, this redirect might take place at different levels.
The web browser hits Apache first. Apache can store redirects in both the httpd.conf and .htaccess files. It sounds like you're hosting in a shared environment, so my bet would be the .htaccess file.
Apache then passes the request to PHP and Drupal. A four-line PHP file can perform the same redirect, as shown here. http://www.cyberciti.biz/faq/php-redirect/
I checked the headers when accessing the home page with the WWW prefix http://www.efg-zoar.de/. This shows the header X-Powered-By: PHP/5.3.10. This makes me suspect that http://efg-zoar.de/ is hitting plain Apache with no PHP or Drupal involved. Here's the header from http://www.efg-zoar.de/.
(Status-Line) HTTP/1.1 200 OK
Date Thu, 02 May 2013 22:02:30 GMT
X-Generator Drupal 7 (http://drupal.org)
You should contact your web host and ask them how to change the Apache redirect in either the .htaccess or httpd.conf file. Sometimes web hosts allow you to FTP the .htaccess file, but you need to configure your FTP client to show hidden files (file names preceded by a period, like .htaccess).
Here's the HttpFox extension for Firefox. It's helpful for debugging these issue.