I have a simialir problem with my work PC on some forums.
If there is a little red 'no-entry' sign in bottom right corner of I/Explorer window, its worth double clicking it, highlighting the url of your forum, clicking summary, and selecting Always allow this .... Once done, re-open site, log in - if you have an option to stay logged in, and stay invisible (not be in active users list), worth turning them off/on and seeing if any difference in browser behaviour happens...