MVC :: Website Gives Only Blank Pages When Deployed?

I'm finally at the point of deploying my ASP.NET 4.0 MVC 2.0 -based web application to the target system and I am getting blank pages for everything - even for the login page. The web application is starting up cleanly without exceptions according to the logs, but none of my MVC routes are ever followed. Instead I get a 200 OK response with an empty page. Here's my environment:

The web application is the root application of a web site. This web site is associated with its own app pool. The pool is set up for .NET 4.0 with a specific local non-privileged user (not the default Network or System user), and this user has read access to the entire web site's tree.I am including System.Web.Mvc as a "Copy Local" reference, which is then in the bin area of the site. I have confirmed that the installed .NET 4.0 already has version 4.0 of the Abstractions and Routing stuff.The web site directory is in the Program Files tree, but any data the application reads and writes is in a different tree, a subdirectory of ProgramData. The app pool's user has read/write access to everything in that subtree. Evidence: the program is logging successfully to a directory in that tree.The site uses HTTPS but I have verified via trace.axd that it is getting through that stuff OK.

Here's what I've seen so far that leads me to suspect a routing problem:

I have a common base class for all my controllers that provides them with common resources. I have a log statement in its constructor, so I should see that in the logs if any of my routes are hit.Nothing is logged if I just give my https://systemname:81/.If I do https://systemname:81/Home/Index, which should be equivalent, I get a blank page from Firefox with a 404 error. (From IE, I get the standard IE 404 page - equivalent meaning.) I do not get the logging in that constructor.If I do https://systemname:81/Home.aspx/Index, I get the ASP.NET default 404 page and no constructor logging.However, if I do https://systemname:81/Home/Index.aspx, I still get the ASP.NET default 404 page, but my controller base class constructor is executed, as evidenced by the log message.