Hi Markus Berchtold,
Indeed it looks like the RadPanelBar output has generated some non-escaped ampersands, thank you for noticing that and letting us know. I have logged the issue in our bug tracking system, and we'll research the reasons behind it. Your Telerik points have been updated accordingly.
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