First, make sure background rendering is turned off.
Then, delete all render files.
Only then would I trash the pref files.
I’ve seen this once or twice, but many years ago.
What macOS and FCPX versions are you running?
Do you have the Chrome Browser installed?
Also, try creating a new Library, new Event, new Project. Then in the original, select all and copy. Then paste that into the new Project.