The uiquery plugin bgiframe (version 2.1.1) causes the following java script error in Internet Explorer 9:
jquery.bgiframe.js, line 94 character 5
After some more research I found a solution myself on the bgiframe site:
I downloaded version 2.1.3-pre at
https://github.com/brandonaaron/bgiframe/blob/master/jquery.bgiframe.js
and this solved the problem.