Hey, man. I had this same thing happen in a “healthy” sequence AFTER I installed some new “aftermarket” plugins. Here’s how I fixed it… (I’ll warn you, this is a less-than-ideal solution, but it fixed me right up and I was able to finish and render out my sequence):
1) Dig in to your plugins folder: /Library/Application Support/Final Cut Pro System Support/Plugins
2) “Dump” all your plugins except:
Vector Shape.bundle
Text Scrambler.bundle
Title 3D.bundle
Title Crawl.bundle
(***NOTE: You can just move them to a new folder or an external & reinstall them later***)
3) Close out your FCP (If it’s open) & then reopen the sequence
4) Voila! It should work (At least, mine did)
Anyway, I hope this helps you or someone else. I would be curious to see if you could just dump the “problem” plugins, since some of my aftermarket plugins still worked fine. I dunno. Again, hope this helps.
God bless & happy cutting,
– Joshua