I am afraid that it is really hard to say what is the cause for the experienced problem without being able to examine the problem at our end. Can you remove all CSS styles from the page holding the Editor and see whether this will make a difference?
Otherwise, please, isolate the configuration needed to reproduce the problem into a runnable sample
and send it for a further investigation by us.
Do you need help with upgrading your ASP.NET AJAX, WPF or WinForms projects? Check the Telerik API Analyzer
and share your thoughts.