IEs debugger is crap. It is hard to tell from that information where the error occurred as it is debugging the output html and not the source code. It could be a Joomla error, mambot error, fireboard error, template error, seo/sef error, etc. You could try googling "'href' is null or not an object" and see if any of the hits offer any insight. You could try using Firebug, which has some support for IE (preferably used through firefox).
www.getfirebug.com/ to better find what the actual problem is.
