I’m also having major issues with EndNote X8 since a recent Sierra update to 10.12.4.
Endnote constantly crashes making work almost impossible. These crashes can take place every time I try to open a PDF or click on a reference! I am constantly having to force quit and reopen, which doesn’t resolve the problem. The issue is also occurring with past back-up versions of my library.
I’ve been using Endnote (Mac) since 2014 and have never experienced anything like this before!!
This is incredibly frustrating, and if not resolved soon, I am going to seriously consider using another program!