Based on the ID attribute id='ai1ec_style-css' that stylesheet is for All-in-One Event Calendar plugin.
If you just updated the All_in-One Event Calendar (the most recent update was yesterday 2013-8-16) try re-saving the calendar theme options in Events->Theme Options.
This answer is based on this support topic. http://wordpress.org/support/topic/broken-links-inside-plugin-after-updating-to-196