I have to emphasise this possible solution if you come to a problem with ASP.NET 4.0 Web Form Routing.
I guess there might be other reasons but in my case it was just this tiny bit:
Enable HTTP Redirection Feature under
Windows Features -> Internet Information Services -> Word Wide Web services -> Common HTTP Features
I had to spend many hours to find the solution, thank you clever guys from Microsoft :-) he he
Solution found here: http://forums.asp.net/t/1523639.aspx by Remy Blaettler, thanks Remy.
Tags: IIS 7.5, ASP.NET 4.0, MapPageRoute, RouteTable, Blank Page, 404, Visual Studio 2010, 2010, madness, suicide