Can't share: frame cannot be rendered

(OP here.)

I appreciate the help. In the end, I wasn't able to get it to work. I did find Finder > Services > Encode Video Files, so I used that, but it took 24 hours and still didn't finish, and eventually I gave up. (It's a 2-hour file.) It was clearly creating such a massive massive file that I wouldn't even have had the disk space to store it.

I've now worked around it using another method. (Note: quality was not extremely important to me; this was a screen recording of a Zoom capture, so quality was already compromised at the source.) I'll detail it here in case it helps anyone in the future (and I'd welcome input from @Tom or others more knowledgable than me) -- but I was desperate!

1. I opened the FCP library bundle using Show Package Contents.
2. I located the problematic file in the Original Media folder, and opt-dragged that file to a different hard drive.
3. I transcoded that file using Handbrake. I tried to use one of the higher-quality presets. This transcoding took about an hour.
4. I imported that new Handbrake-produced file back into the FCP library.
5. I went into my multicam clip, chose Add Angle, then moved the problem clip to the newly created angle, making 100% sure that the start time was the same.
6. I now imported my Handbrake-produced fixed file into the multicam clip, and put it in the lane of the original angle, again, making 100% sure the start time was the same. I could also verify this visually since the waveforms gave me instant feedback that it was in sync.
7. Since I had now replaced the source file at the multicam clip level, all the edits at the project level now automatically inherited the replacement file, and I was able to finally render the finished project without error.

Phew.

Unfortunately none of that worked.

I started by using the File menu to "delete generated files" and made sure to click ALL render files.

But then, after quitting, I used Show Package Contents and still found a large quantity of render files inside the FCP library. So I manually moved them out. Then I reopened FCP and trashed its prefs (via launching it using opt-cmd). Then I reopened the library, and waited hours for it to do all its re-rendering. Then I tried another export: it got about 80% of the way done, then gave me another similar error, at another random point.

I have no idea what to do now...

Question: it seems like all the errors have to do with that specific file ("2021-01-31 BTTF Episode 2 SF Export.mov"). What if I go to the Finder, find that file, and find some way to resave it (perhaps through a relatively harmless Handbrake transcode, or a QTP re-save), then I reimport that to my library with a different filename, then try and swap it out? I wonder if I can swap it out at the Multicam clip level, thereby preserving all the thousands of edits that I've done at the project level...

Thanks Tom. I'm not sure I fully understand. Would you mind spelling it out for me?

How do I "convert the file"?

Then in order to relink it, are you implying that I go into the FCP library package, remove the old one, relaunch FCP, wait for it to complain about the missing file, then relink to my newly converted version?

How do you "use the Finder to transcode the file"? Right-clicking on it does not offer an Encode option.