To circumvent this silliness and get real ASP errors, go to IE's Tools/Internet Options menu, and on the advanced tab, uncheck "Show friendly HTTP error messages."
Hit Apply, OK, and refresh the page in question. There are five possible outcomes; the page will either:
1. still yield cryptic and unhelpful error messages (e.g. Page Not Found);
2. stop processing where the error occurs, and leave the rest of the page blank;
3. return "Server Application Error";
4. give you a more detailed error;
5. magically work again.
For (a), open Internet Services Manager, go to the home directory tab of your default web site or application, click on Configuration, go to the Debugging tab, and make sure "Send detailed error messages to the client" is selected.
Page not found errors can also be caused by the IIS Lockdown wizard; or, in Windows Server 2003 / IIS 6.0, a misconfigured Application Server. See Article #2147 for information on configuring your application server to allow the server to process ASP files.