Getting 503 errors on IIS 7 with custom application pool (Integrated Mode)?

So today I’ve though about giving IIS’ integrated mode a shot…The idea was simple: create a new application pool, configure it to use ASP.NET 4.0 in integrated mode and then associated it to an existing web application. The problem: The first request would always kill the application pool and it returned a 503 error to the browser. IIS Manager seemed to confirm that the pool was really dead:

WTF? I’ve read a couple of posts on IIS 7 and 503, but none helped. Then I tried one more thing: I’ve configured the web app to use the default ASP.NET v4.0 application pool. And it worked…the mystery was solved when I looked carefully at the application pool list. Can you spot the problem? If you’re thinking that my custom livro application pool is using the wrong .NET version, you’re right! I don’t know how it happened, but it seems like all this beta stuff I’ve been adding to my machine removed the correct version of .NET from the .NET version dropdown (I only had v4.0 available from the dropdown list).

Running aspnet_iisreg from within the 4.0 framework folder made everything go back to normal. So, are you getting 503 in integrated mode? Don’t forget to check the .NET version used by the application pool…it might not exist and that means 503!