Please take a look at Sitefinity's UserManual, section Extensionless URLs.
Possible reasons for this error are:
-Wrong setting of IIS for extensionless urls.
-There is a something else on the website that misleads the url parser.
sf404 should be configured in IIS as Error page/WildcardMapping or in the web.config as custom error. This is the correct setup. If the problem persists you can try setting setrunAllManagedModulesForAllRequests
attribute to true in your web.config
the Telerik team