EndNote X9 stopped working on my Mac (Mojave). It did work before, but now it won’t even start: The app icon bounces in the dock for a while, but it never shows a window or menu. I already tried restarting the Mac, reinstalling EndNote, and removing the EndNote folder from ~/Library/Application Support. Nothing helps.
Try starting in safe mode and see if it still happens. Restart the Mac with the shift key held down (it will take longer).
You could also try deleting the plist file. This is a preferences file, and EndNote will recreate a new one if needed. Hold down the Option key and click on “Go” Finder menu. Click on Library. This will open a window - have a look down near the bottom for com.ThomsonResearchSoft.EndNote.plist