My experience too. I haven’t found a solution that holds up longer than a short session, then quitting Endnote, and restarting it: repeat. I wondered if it was my machine, but it is fully loaded (MacBook Pro 2016, 16 GB RAM, 1TB storage).
The problem is with EndNote, as others are attesting.
PS: Don’t follow the advice of their help staff, who will make you run through endless hoops that do not avail, like relocating your library, rebuilding it, etc.