We did some testing today and discovered that mootools version 1.2.4
SCRIPT5022: DOM Exception: INVALID_CHARACTER_ERR (5)
It is visible in joomla admin and in frontend if you have any mootools based scripts including all our sliders, and Yj Mega Menu for Joomla 1.5
This post from Joomla org
Joomla! • View topic - Mootools 1.2.4 and IE9 Bug
advises to update the Joomla 1.5 core mootools upgrade plugin to version 1.2.5.
We have tested the advised mootools pack and it works.
Since this is Joomla core issue , we cannot perform update until they do . So for the time being please download the file attached and replace
We have tested as much as possible and did not find any issues. Please do advise if you have trouble with this file.
Note that issue is visible only if Mootools upgrade plugin is on and you are using IE9 browser