Symptom:
After adding the “Capture Reference” bookmark from EndNote Online, every once click on the bookmark will always showing this “Log into EndNote” in a loop, despite already entered correct username and password.
The official FAQ ask for reinstall the plug-in which is not helpful.
Solution:
Turns out it can be resolved by un-ticking the “Prevent cross-site tracking” in the Safari setting (cmd+,) -> Privacy -> Website tracking.
Click the bookmark and login, now it works!