Can you, please, tell me if the issue reproduces with newly created pages as well or on a new blank project? Generally just by changing the pageExtension=".aspx"
in the web.config <cms> tag should work just fine, and your pages should be extensionless - can you try testing only with this setting? Also, can you make sure you are not specifying any additional URLs for your page that are extensionless already, e.g. you have ~/MyPage.aspx, but you have specified additional URL of ~/MyPage, since this might create confusion.
the Telerik team