Hello Markus Berchtold,
We just checked this issue and it seems that we had an issue with auto generating links for reply to a thread.
The good news is that the issue will be resolved on our next planned web site maintenance that will happen in a week or two. Until then please temporary don't use the provided link in our emails.
Please take our apologies for the inconvenience caused and don't hesitate to contact us for any further issues that you experience!
All the best,
the Telerik team
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