I found that this bug comes from the RadCompression preCondition that was seted on "managedHandler" instead of "integratedMode" for the following reason:
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.
This problem is caused by a breaking change in .NET 4.0 described here. To workaround it one can set preCondition="managedHandler" for the RadCompression module. You may also need to remove the runAllManagedModulesForAllRequestssetting from your web.config if you have it (or set it to false).
So if there is a solution to have both the default pages and the radtextbox working, I am interested to know it.