Ive ran into that same problem with a few select users. I told them to change their security settings, delete temp files, and reset cookies. Nothing worked. They had the latest IE version as well. The only solution which worked was downloading Netscape communicator.
I do not think it's a problem with the forum, though. I believe it's a bug within IE itself. Over the years I have always found Netscape to run Java and javascript much better than IE (Probably because they invented it, so they should run it better). Probably just one of those things you have to live with.
If you do find a solution please post it here as I would be very interested to see just what you did.
Blade