I currently encountered this issue on a Mac with Endnote X9.1 (also tried Endnote X9) on MacOS Mojave 10.14.3. A default install without selecting custom settings. Usually I have seen this occur randomly on several versions of Endnote on Mac, but here it was all the time. Very frustrating.
On this Mac iCloud drive was disabled, but to be sure an empty library was created in the user’s home folder. We did all the testing with .enw files trying to import those. I usually also make a file association by selecting cmd+i and setting Endnote as the default program, otherwise you get an annoying pop-up to select the default program and open the library every time, even if a library is already open and you ticked the “default program” box.
Anyway, we tried a lot of different approaches. I made sure to create an empty library to test with, even created one on a memory stick with the same “This library is currently being used by someone else” message. We tested with both Safari, Chrome and Firefox. The latest versions of those. Firefox is usally a nice solution, because it can open .enw files directly with EndNote, but this time we got the same error message.
I know this user had EndNote sync turned on at first install, but I made sure to remove all preferences and reinstalled EndNote and made sure that sync was turned off.
This mac was quite clean, but McAfee antivirus (which I dislike) was running on it. Tried to disable it, but the same error popped out. A 3rd party cloud service to make backups was also installed, but not running. Really do not know what else to try?