The checkerboard error indicates that Final Cut Pro cannot find/access some of the media files. The problem results from storing Motion Content in the Library and not in the Movies > Motion Templates folder – in this setting, Final Cut Pro copies the folders containing projects that are on your timeline from the Motion Templates directory into the Library file. For some reason though the app doesn't copy our global Media files stored in a directory above, which results in the checkerboard error.

To fix this issue: go to Library settings (Ctrl+Cmd+J) and modify Storage Locations changing Motion Content back to the Motion Templates folder. Restart Final Cut Pro afterwards.