I have been fighting all week with IIS and ASP. The server is a Windows 2008 R2 machine running IIS 7.5.
I kept getting 500 Internal Service Errors.
I've been going back and forth with the developers, creating app pools, resetting things, adding new lines of code, and pulling my hair out in between.
Today I happened across the blog of Don Draper. Don has a blog post up about getting IIS to give you detailed error messages. This was exactly what I was looking for. After following his directions, I hit the ASP app in the browser again and there it was - exactly what the hangup was. I am not going to reproduce the steps here, as Don has done a great job on his site. As a side note, you definitely want to be careful with this. I would not enable detailed errors on a production app.