Hello Andreas Kaech,
There is one difference between the old PageViewItemCreated event and the new PageViewCreated. The old event was fired only after postback whilst the new event is fired always (even on initial load). I presume you are creating the editor outside the PageViewCreated event handler (as was suggested in the "old" way). However now the editor will be created twice hence the duplicate id error. If you instantiate the editor only during PageViewCreated you should avoid that error.
the Telerik team