Could anyone confirm the error on this page (http://www.ukagriculture.com/crops/oil_seed_rape.cfm) when you mouse over the horizontal DHTML menu, I've updated the page with OpenGraph data to keep the facebook script happy on page load, I now have the menu activation JS error.
Are you using IE8? My menu system works OK in Chrome and firefox.
I can get the same error on this site: http://software.xfx.net/ when I use the menu system. Seems its an issue with facebook code and menu code existing on the same page. If I remove the facebook code the menu system works. I've tried 3 types of "like" link but all cause the same issue.
Thing is nothing changed my end, just starting playing up!!
Can you please post line 154, and also post any code related to it, such as the lines which call any functions that are in question. Also, does this happen to you in all browsers? I'm guessing this is an old version of IE?
An invalid argument is obviously referring to one of the function parameters, but which one I can't see. I know of other places where this error can show, for example using the window.open() function in old IE versions will not allow spaces to be present in the windowName parameter (2nd parameter).