I tried to open my original version of Calibre, but although the dot appeared below the icon, nothing happened. I downloaded a new copy, but when I tried to run it my Mac said Calibre was already running. I got this message:
calibre, version 3.35.0
ERROR: Cannot Start calibre: <p>calibre is already running.</p><p>calibre may be running in the system tray, in the upper right region of the screen.</p><p>If you are sure it is not running, try deleting the file: "/Library/Caches/calibre-singleinstance-501-GUI.lock"
Timed out waiting for response from running calibre