need help-some sort of error

My members have been complaining about a Microsoft promt error poping-up and closing the site.

I am not sure where to start inorder to fix this problem, any ideas?

Site is here- http://www.t-og.net and apparently occurs when they try to make a post and view the forums.

Any ideas or further information needed please reply, im dying for some help
cheers

p.s. this only has happened on internet explorer and doesnt happen on firefox. Also this doesnt happen to me... ty.