I had, I think, the same problem and after 2 hours of frustration found a solution that worked.
In the properties of EndNote 20 application file, allow ‘Write’ in security settings.
C:\Program Files (x86)\EndNote application file - right click - properties - security tab - Edit - tick ‘Write’ box in allow column - apply - ok.
I don’t know if this will work for others, but thought it worth sharing.