Using Confluence 5.5.4 it was working and just stopped. It works when in a new page. Also if I click to edit a page that fails however, if I take the URL and copy it and paste it into a new browser window it works until I try to save and reedit the document.
This is happening with all three browsers IE, Firefox, and Chrome.

Go into any page with edit or at any comment the // simply brings up a "/" instead of the calendar.

It appears to do something as the font does have a grey shading but only another slash "/" not the calendar is brought up as per the attachments.