As Andrejus said, you need to ensure that you're using the most recent version of the ExtLib everywhere you're using XPages.
The stack trace implies that your Designer install is using a version of ExtLib using a property that was introduced 853-20111117-1058, but that your runtime environment is using an older version of the ExtLib where that property is not present. So you would need to upgrade the ExtLib version in the runtime environment.
Your runtime environment might be a Domino server, a Notes Client install (different to the Designer install), or a Designer Web Preview server - when you install ExtLib into Designer you also need to separately install it into the Designer Web Preview server.
[Instructions for installing to the Designer Web Preview server: http://www-10.lotus.com/ldd/ddwiki.nsf/dx/Installing_an_XPages_Library_into_Designer#Preview+in+Web+Browser ]