Links:
[1] http://www.lib.uts.edu.au/vote/node/5930/1/vote/utslibvote/Ry4CZmQQ5ZF8HO3LyO2_FB72TFL5pk3jMKTlhxixtyE/nojs
[2] http://www.lib.uts.edu.au/help/referencing/endnote
[3] http://www.lib.uts.edu.au/question/5930/invalid-class-string-windows
[4] http://www.lib.uts.edu.au/node/5930/comments/newest
[5] http://www.lib.uts.edu.au/node/5930/comments/oldest
[6] http://www.lib.uts.edu.au/comment/222#comment-222
[7] http://www.lib.uts.edu.au/user/uts-library
[8] http://www.lib.uts.edu.au/vote/comment/222/1/vote/utslibvote/WbbrENlOIzUMd6pzTnha6SVJqPUp-zw26B5NcH7HS5o/nojs
[9] http://www.lib.uts.edu.au/question/5912/wheres-word-2003s-endnote-toolbar-windows
[10] http://endnote.com/support/faqs/endnote
[11] http://www.lib.uts.edu.au/user/login?destination=node/5930%23comment-form
[12] http://www.lib.uts.edu.au/tag/endnote
[13] http://www.lib.uts.edu.au/tag/endnote-error-messages
Answers
This is probably because you [6]
This is probably because you have recently upgraded EndNote and the old version has not uninstalled properly. In particular, you probably still have the old version's add-ins, which means that your EndNote toolbar in Word still belongs to the old version and gives this error message whenever you try to use it.
There are several options: probably the simplest is to uninstall both the new and the old versions of EndNote (again), and reinistall the new version. If this does not solve the problem, another option is to go to Word's startup folder and make sure that the two add-ins there are the correct ones for your versions of EndNote and Word [9].
Finally, there is some guidance under CWYW 13 on EndNote's FAQ site [10].