I've (eventually) found that activating My Calendar results in 404s for permalink redirects to custom page types in the themes on a couple of my sites http://www.pavilionphotographicstudio.co.uk and http://www.yerburygallery.com/ (warning nudes - NSFW).
Both are running themes from http://themeforest.net/user/doitmax
It also breaks the dynamic site map generation from the Yoast SEO Plugin.
Breaking it is easy - just enable the plugin and the redirections break.
Clearing it is harder - I've found that I need to remove .htaccess, reset the permalink settings and reset the Yoast SEO site map settings too.
It looks like this broke with one of the updates in mid-December (that's when my site map broke on the Google webmaster tools).
I'm happy to help debug this...