Hi Everyone,
I have had the same issue as described in this thread.
I was able to get Endnote X8.2 to work as normal with Office 16.9 (Mac) by removing the citation plugin for Mendeley.
I guess there is some conflict between the update for Mendeley which was release to work with Office 16.8, and the Endnote update. Removing the plugin has been a fine fix for me, as I only really use Mendeley to organise my papers.
Hopefully this will help other with the same software configurations.