thanks a lot for your answer!
Of course you're right that the exception is about the WebResource.axd and not the Telerik one. My fault I forgot to tell you that I already tried that before, but since it did not work deleted it again:
|<add name="WebResource_axd" path="WebResource.axd" verb="GET" type="System.Web.Handlers.AssemblyResourceLoader" />
I thought it was not needed since there is a handler (named "AssemblyResourceLoader-Integrated" and of type "System.Web.Handlers.AssemblyResourceLoader") in IIS on server level. The web.config entry is also not present in the demo I mentioned above neither in <system.web><httpHandlers> nor <system.webServer><handlers>.)
I tried again adding this handler to my web.config and as expected it still did not work.
But I found the solution to my problem in the thread of the link you posted: I am running ColdFusion 8 in the same application pool as ASP.NET and the wildcard handler needed for CF is registered. According to the discussion in another forum thread
it should be fixed in .Net Framework 3.5 SP1 but obviously is not since I get the error with SP1 being installed and also others are reporting same problems.
Anyway, after I removed the wildcard handler (named "AboMapperCustom-1169055" and pointing to "C:\ColdFusion8\runtime\lib\wsconfig\1\jrun_iis6_wildcard.dll" in my case) the RadScheduler Control now appears without any errors!
And you were totally right, the web.config entry for the WebResource.axd handler is really needed. Without it I get the same exception again..
I guess the ColdFusion applications won't work now that I removed the wildcard handler, so I will try to separate ColdFusion and ASP.NET applications tomorrow..
Thanks again for your great answer!