HTTP Error 503. The service is unavailable under simple ASP.NET 4.0 web site

There are subtle changes between ASP.NET 2.0 and 4.0 regarding application start up. For instance, you may not access the HttpContext object during the Application_Start event in ASP.NET 4.0. Do you have any code that might hide an exception being thrown because of this?

There are a few problems that may cause the AppPool to stop. One which I've run into myself is that any unhandled exception on a thread other than the request worker thread will cause the AppPool to eventually stop. This is not an immediate problem but eventually it will stop. The ASP.NET runtime keeps track on how frequently your app is failing and if it breaches that threshold the AppPool is stopped, taking down with it, any applications sharing that pool. A StackOverflowException or OutOfMemoryException will eventually have the same effect, these are critical errors and shouldn't be happening in your everyday production code.

I would review the changes between ASP.NET 2.0 and 4.0 and look for unhandled exceptions. You can also change the way Visual Studio handles exceptions (check under Debug > Exceptions) and break when they are thrown regardless if they are handled or not, this is a quick but very verbose way of finding any exceptions.

I've reinstalled .NET 64 - that helped.

P.S.

It seems like either some files in "C:/windows/Microsoft.net/Framework64/v4.0.30319/" folder or I removed them myself (VS sometimes complains on files in "ASP.NET Temporary files" and their deletion helps)... Probably I didn't pay attention that those folder not a temporary...

P.P.S.

In this case, why VS complained on files in "C:/windows/Microsoft.net/Framework64/v4.0.30319/" folder... ok, now it's hard to say.

Sounds like you forgot to enable .Net 4 extension on the IIS. Try finding and enabling it in ISAPI and CGI Restrictions

http://blogs.msdn.com/b/rakkimk/archive/2007/08/17/iis7-where-is-the-web-services-extensions-option-which-was-there-in-iis6.aspx

Based on the comment it might be that part of the framework was removed and in that case it might be wise to reinstall Framework 4 by first cleaning it up. Try this blog post which got a reference to a tool that automates cleaning up proccess http://blogs.msdn.com/b/astebner/archive/2008/08/28/8904493.aspx

I had to start the application pool that was set for my website and was automatically stopped on some error. (IIS (7.5 in my case)->Application Pools->Start stopped application pool.)