The problem was that the template has to be placed in the DisplayTemplates-Folder, as it is written in the msdn.
If a template whose name matches the templateName parameter is found in the controller's DisplayTemplates folder, that template is used to render the expression. If a template is not found in the controller's DisplayTemplates folder, the Views\Shared\DisplayTemplates is searched for a template that matches the name of the templateName parameter. If no template is found, the default template is used.