The support and development of RadEditor Classic (RadEditor.dll) was discontinued in 2009 and it does not support the modern browsers including IE9. That is why my suggestion is to render the page in IE8 mode using the following meta tag:
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE8" />
If the problem still persists, migrate to the new and much enhanced RadEditor for ASP.NET AJAX following the instruction in this article: Migration from RadEditor Classic to RadEditor for ASP.NET AJAX
. The latest Q2 2012 build offers full support for all modern browsers: IE9, FF13, Chrome 20, etc.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed