And if you are having problems with a particular Event the same advice goes. On Finder level delete the Render files on that Event folder. Hope all goes well!
First shut down FCP X. On Finder go to your Project folder and locate the Render Files folder. There are three folders: High Quality Media, Peaks Data and Thumbnail Media. You can trash all of those if you wish, but first delete the “High Quality Media” folder and start the FCP X.
You can also use the Preference Manager app to trash your prefs. It’s free and highly recommended to have: https://www.digitalrebellion.com/prefman/