Just an idea: are you trying to post a link to a video file? The forum software automatically DOWNLOADS the entire file (rather than getting just the header to see if it's HTML and *then* getting the page) to determine the title for the link. If you do something like
[url=http://address.to.video/goes.here]Link title goes here[/url]
it should work, because the forum will then not attempt to download the file. Just don't paste the address without url-tags and a title.