The reason for the experienced problem is indeed a know issue related to the usage of RadCompression. It's mentioned in the following help article here
under the Known issues
When RadCompression is enabled and you are using .NET 4.0, event handlers might not be raised in a default document in IIS 7 or IIS 7.5 Integrated Mode.
Please try the solution suggested in the aforementioned help article.
the Telerik team
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