app_offline.htm throwing HTTP 500 errors on production box

I have created an app_offline.htm file for an ASP.NET MVC2 application running on IIS7 / Win2008 64-bit, and ensured that it's over 512 bytes (it's 2KB right now). On my dev box running Visual Studio 2010, it works like a charm, but when I put it on the production box, all I get is the generic HTTP 500 error saying "The page cannot be displayed because an internal server error has occurred."

What's especially strange is that I don't get anything logged in the application event log, nor does ELMAH pick anything up. I've disabled custom errors, put FormsAuthentication location exceptions for the file, ensured I'm not referencing any other files (images, etc.), but nothing fixes it.

I've read every post on SO and Googled for hours and can't figure this out. Any ideas what might be wrong? I'm pulling my hair out here...