I had this same problem and spent an hour on the phone with Final Cut support and we finally figured out to fix the problem we had to trash both the Thumbnail Cache and the Waveform Cache.
This is found under the user preferences on your hardrive – mine happen to follow this path:
my hardrive/users/fcp_user/Documents/Final Cut Pro Documents/
– here you should find the “Thumbnail Cache Files” folder and the “Waveform Cache Files” folder
– drag them to the trash and make sure to empty the trash
– Do this while Final Cut is not open
– Launch Final Cut and it will rebuild the image and waveform cache which should solve the problem
Hope this helps!