It seems this is due to an incomplete vBulletin feature from vB 4.1.0 - 4.1.7 (it was finished in 4.1.8). Users on any version in that range would receive this error when attempting to see AJAX previews and possibly some other AJAX features.
I have made the appropriate changes to the product for 3.0.17 so it will work on those versions too.
While I was looking into this, I noticed some issues with HTML on your site, that you may want to look into.
1. In your headinclude template, you have added a script tag with no type attribute, and I assume you mean type="text/javascript".
2. In your headinclude template, you have added a style tag with no type attribute, and I assume you mean type="text/css".
3. In your headinclude template, you have added a number of tags (div, p, br, etc) that are not allowed in the document head. That code should be moved to another template that is contained inside the HTML body tag (templates like header, navbar, or footer). Also note that </br> is not a valid tag.