Its all ok with your code. When i started debug you code i saw some interesting thing in alertbox.js: (line 141 and 178)
if (!$.support.maxHeight) { //IE6
$('embed, object, select').css({ 'visibility' : 'hidden' });
}
This code detect ie6(if read a comment) but seems its buggy.
Just comment these lines and your problem will be solved.
And don’t forgot to post this bug!
Good luck. And start use debugger 🙂