[Plugin: WordPress.com Stats] WordPress Stats broken

I use one API key for all of my own WordPress sites (self-hosted, not hosted by WordPress) and all of my clients’ sites. Today I noticed that ALL of the WordPress stats on all the sites are not working anymore. It says that my WP username is not authorized to view the stats. I have logged in and cannot see the sites in my list anymore. They’re gone. I can also not add or edit the API key in any of the site admins.