A number of people have reported problems with IE 6 in the past. The problems usually caused by compressed web resources (WebResource.axd and ScriptResource.axd from ASP.NET 2.0 and ASP.NET Ajax Extensions). IE6 has problems downloading these resources. Try getting the latest critical updates for IE from Windows Update and see if that helps.
If there is no error during page load and you have the latest from Windows Update, then please send us a sample master/content page so we can test them locally.
I also did a simple google search about this error but the results were too general to be of any use. If possible, use a HTTP debugging proxy (for example - Fiddler) to check exactly what requests are made by IE and which one fails.
the Telerik team