I've been putting together a website for a few weeks, most of the routine has just been creating new web forms with master pages. I've been using the ASP.NET Development Server to view my additions to the project as I've gone along, but today (seemingly out of nowhere) the server has been returning HTTP 503 errors. I tried restarting VS, then restarting workstation, then making sure that "bypass proxy for local addresses" was enabled on my workstation, but to no avail. Has anyone encountered this behavior before or does anyone have suggestions on logs I can review to investigate? Thanks in advance.
Update: When using telnet, the server returns the markup for what looks to be an ASP.NET server page, listing an HTTP Error 400 - Bad Request. Still getting a 503 in Google Chrome and blank pages in Firefox and Internet Explorer.
1) Maybe there is errors in web.config, so cassini can't understand it. Try to publish website and run it with normal IIS
2) Remove file app_offline.htm from root of your project. It's really generate 503 error with cassini
did u tried enabling debugging in your web application?
http://msdn.microsoft.com/en-us/library/e8z01xdh%28v=vs.80%29.aspx
One possibility is that some other application is now configured, say, in IIS, to listen on the same port/vdir combination. It might be a port conflict between two site/servers/applications. I would get Fiddler tool to look at response headers and see whether you are getting the response from the server you expect.
来源：https://stackoverflow.com/questions/6631368/what-reason-would-visual-studio-asp-net-development-server-start-returning-http