I spent several hours looking for why WP Super Cache didn't work on my blog and finally discovered that it was because of the AskApache What-Is-This plugin (version 2.1). I'm posting this because I hope this info could be useful to other users that may have a similar problem.
The WhatIsThis plugin does
die() in it's main function (that hooks wp_footer) if the visitor doesn't have admin rights. This can cut off the last part of the theme's footer, including
</html>. In turn, WP Super Cache won't cache that page because it is vary of "incomplete" pages and checks for the presence of the
I've already notified the author of the WhatIsThis plugin about this.