Photos Cannot Play This Video Because the Resource is Unavailable After Catalina Update

1. FWIW when i select a problematic video within Photos, and go to Image-->edit with-->quicktime, I get this message: "An error occurred while preparing external edit- Failed to download CPLResourceTypeMediaMetaData for AY0ZIF0HRYsstRIQWu4pJ6rEq5fA#PrimarySync (Asset field not found)."
2. Some videos that weren't playable seem to be playable now within Photos. I'm not sure if it was the rebuild/repair or something else that is going on while the computer now uploads all my newly repaired library into the cloud, but I suppose that's progress? Whats interesting is that some videos are available others are not AND some videos that were available a couple hours ago no longer are.

Update: Tried rebuilding/repairing library. Does not seem to have immediately rectified the situation...still getting same message "Trying to download 218 etc.".

On top of that, there are now a bunch of photos showing up as "Unable to Upload" for whatever reason. These are files that were already on my computer, and in the cloud, taken somewhat recently. The number of photos is growing. When I log into my iCloud browser I see those photos still available in my iCloud account..

Hi, the unable to upload photos all seem to be compatible file types. In fact, many were shot recently on my iPhone (11 pro). Others are png files, etc. Nothing that should be incompatible it appears. Also, these files already seem to be in the cloud (I see them when i look at iCloud through a browser).

Re the resource not found issue, I'm not sure what's going on there. Still have the issue after rebuild/repair.

I use one library and that library is the Pictures folder on the iMac itself, not an external volume.

Good point regarding iCloud. Perhaps there are files up there that are unplayable on my Photos app. I don't know how Gopro videos, for example, (4k HEVC MP4) would fall into that category, though.

Don't believe it's a permissions problem. I'm waiting for Photos to "upload" to the cloud after having rebuilt/repaired the library. This process will take a few more days, minimum, I gather.

I'm hopeful it will resolve matters.

FWIW the creative support team at Apple recommended I just scrap this library entirely and create a new one. So basically download my entire library from iCloud into a new library on my mac. Then delete the old. I tried the first part (download the full iCloud library into a new library on my mac, optimized for storage). I then tried to play a full size version of an optimized GoPro video and go the same source message soooooooo for now I'm going to scrap the new library plan and let the old continue to upload to the cloud, in the hopes that the repair/rebuild has fixed matters.

Forgot to add:

Exported video unmodified video to desktop. Opened In quicktime. Exported that video to desktop in 4k using a new file name. Importaed that video (now MOV not MP4, as it was prior) into photos and it played. 

Im still in uploading purgatory after having repaired my library. About 80% done, it appears.

Interestingly, I'm getting the error message "Resource not found" on more videos than was the case earlier.

We will see what happens.

I tried opening Photos in safe mode to see if those same videos worked. They did not. Same error.

Mikael, thank you so much for your input. Where is the location of this masters.legacy folder within Catalina? I would love to check if the videos I see there are the ones that are giving me this error.

I would add, however, that the problematic videos aren't that old, from what I can tell. They are mostly GoPro videos shot in 2019 and some Nikon D850 videos shot over the course of the last 18-24 months