Did you have an older version of Endnote installed? The only mention I have seen to a similar error, is FAQ 38, which refers to Endnote 4 http://www.endnote.com/support/faqs/CWYW/faq38.asp and an old discussion list item
No, we’ve got the X2 version installed. We once came across a similar problem and thought of an “older” version not being uninstalled as a possible “error source”. It hasn’t caused any problems up to now, but one has to close 3 windows with similar failure information before being able to work with word. Maybe we just have to uninstall the older version…