ASP.NET 2.0 error: Unable to start debugging on the web server.

There may be different causes for this type of Error :
One way of Occurace is due to ... [UrlScan]
UrlScan version 2.5 is a security tool that restricts the types of HTTP requests that Internet Information Services (IIS) will process. By blocking specific HTTP requests, the UrlScan security tool helps prevent potentially harmful requests from reaching the server. UrlScan 2.5 will install on servers running IIS 4.0 and later.

RUN > Search > For Files and Folders > Type - 'UrlScan.ini' (without quotes)
Open the ini file in a Notepad , there will be different section in this file .. such as UseAllowVerbs,UseAllowExtensions ..etc . Check whether .asp is there in deny section. if so delete that..also include that in allow section..then check the AllowVerbs section..add Debug there..

Happy Coding ..