How to configure IIS 7.5 to allow special chars in Url for ASP.NET 3.5

I'm trying to configure my IIS 7.5 to allow specials chars in the url for ASP.NET. This is important to support wide-spread legacy url's on a new system.

Sample url: http://mydomain.com/FileWith%inTheName.html
This would be encoded in the url and requested as http://mydomain.com/FileWith25%inTheName.html

This simply works, when creating a new web in IIS 7.5, placing a file with the percentage sign in the file name in the web root and pointing the browser to it.

This does not work, however, when the web site is an ASP.NET application. ASP.NET always returns a 400.0 - Bad Request error in the WindowsAuthentication module from the StaticFile handler, when pointing to that url. It however displays the requested url correctly and also resolves correctly to the correct physical file (the information from the field 'Physical Path' from the Server error page points to the physically available file).

There are hints on how to enable this, so I followed the instructions on these websites step by step:

• http://dirk.net/2008/06/09/ampersand-the-request-url-in-iis7/
• http://adorr.net/2010/01/configure-iis-to-accept-url-with-special-characters.html

The second one actually sums up the information from the first post and adds some more information about x64 systems (we're running x64) and on an additional web.config change for this. I tried all that, and still can't get this running from an asp.net web application. And yes: I rebooted after applying the registry changes.

So, what do I have to do in addition to the settings described in above posts, to support the legacy url's which contain percentage characters?

Additional info: Application Pool mode is integrated.

Push after some days. No idea anyone?