Hello Markus ,
Can you try and run Firebug, check the Net tab and tell me what error is coming up there and tell me more about your current project setup because on our local environment the feature is working flawlessly.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items