The new standard (IDNA2008) makes it possible to use a ß character. Albeit, not all browsers and mail-clients display domains with a ß (German: Eszett) character correctly. The most programs change the ß into ss. As a result, not the content of the ß domain is displayed but the content of the ss domain.
Since we as domain registrar have no influence on this, an appropriate implementation by the browser developers is necessary.
You registered the domain straße.de. The browser changes the domain to strasse.de. Thereby not your content is displayed but the content of strasse.de's owner.
You can find further information on e.g. Wikipedia: