Here's a couple more pieces of the puzzle:
1. The
login page has mixed secure and non-secure content - if you browse to it you get a warning message (popup dialog in IE, yellow bar in Chrome).
2. If I run the recipe while Fiddler (a web debugger/proxy) is running, the recipe runs successfully.
It's almost as if the mixed content is confusing the BeautifulSoup browser and somehow Fiddler is ameliorating that confusion. But I don't have any idea how to go any further (I suppose I could leave Fiddler running every night for now, but blech!)