Since you are using RadSplitter.Net2.dll this means that you are using RadSplitter for ASP.NET and not RadSplitter for ASP.NET AJAX. In the classic RadControls suite there was a separate dll for each control as RadSplitter.Net2.dll, etc while in the RadControls for ASP.NET AJAX all the controls are embedded in one single dll called Telerik.Web.UI. The classic RadControls were released a long time before IE8 was published and since the trend is to develop RadControls for ASP.NET AJAX they do not support IE8 and only major bugfixes are applied to them. This being said, in order to support IE8 you should upgrade to RadControls for ASP.NET AJAX.
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