Oh, btw, it is the plugin. Multiple plugins. Every plugin that tries to show the spinning busy ball. This is because RC a few versions ago changed the way you have to call that function, and in the process broke every plugin using it, resulting in the sticky loading msg.
Old code:
rcmail.set_busy(true, 'loading');
rcmail.http_post('plugin.get_addressbook', '', true);
New code:
lock = rcmail.set_busy(true, 'loading');
rcmail.http_post('plugin.get_addressbook', '', lock);