I too was having issues with EndNote X7 ‘hanging’ in Sierra (10.12.4) last month.
I got in touch with Jay Nadler (CEO at Clarivate Analytics) via LinkedIn on 22nd Apr. to express my dissatisfaction with X7 on Sierra. Jay recommended the upgrade to X8 and support colleagues got in touch provided a free upgrade for me, which appeared to resolve the issue.
However, since upgrading to Sierra 10.12.5 this issue appears to have reappeared in EndNote X8. Now I ‘WannaCry’ for sure.
Clearly this is a pressing issue for the whole community.