Had this happen a while ago and after searching, calling support and pulling my hair out, I figured it out! I’m on iPhone right now so not sure if this address will work on a non-mobile device but here’s a link to it:
https://mobi.creativecow.net/thread/8/1029264#1029267
The title was something like: “HELP! Wierd problem opening FCP”. That way you can get the while story. Basically the fix was to delete your thumbnail and waveform caches from the folders inside your scratch folders. Now…whenever I open that project again, I have to delete the caches again but at least it fixed it for other projects. I’m assuming that when I deleted a couple of render filed, not using the render manager, it corrupted that project file or something.
Lemme know if that works!
Tony Brittan
Island Shore Productions