Ok, quick update for those who might be struggling with the same issue.
In my case, it had to do with the DocType definition.
We use:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
But changing it to below fixes the error:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
I guess CuteChat is not XHTML compliant (?).
Funny thing is, it only occurs on IE 8, not IE7.
Oh well...