is this forum still active?
Does anybody have an idea or should I open a JIRA?
Sorry Alain, I didn't receive any notification of your original post!
It's not something I've come across before, but then I've never used HighCharts and don't personally test in IE8.
If an RF portlet without HighCharts works fine on IE8, then it's possibly related to HighCharts utilizing some URL paths that RF doesn't. To fix that kind of error may require fixes into PBR.
Can you try with the latest PBR 3.3.2.Final?