undefined error

when trying to update calender from the front end, this message comes up

Message: ‘undefined’ is null or not an object
Line: 1
Char: 533
Code: 0
URI: ymcanb.org/wp-content/plugins/ev … ler_url.js

and then a window pops up asking if i want stop running the script. i am using calendar 2.3.1 on wordpress 3.1.1 and viewing it on IE8.