I can confirm that this issue seems to have been resolved in iOS 14.5, at least when testing on iPhone 11. Also, I've been in contact with Apple Developer Technical Support regarding this issue, and they confirmed that this looks like a bug in AVFoundation, introduced at some point during earlier iOS 14 releases.

I also experience this exact issue when trying to export an AVAsset from a shared iCloud album, where I'm not the creator of the original video file. Duplicating the video from the shared iCloud album to my own device in the Photos app, resolves the issue. It is now possible to export the asset without errors. It seems that AVFoundationErrorDomain Code=-11800 translates to AVErrorUnknown so there is not much information to get from this. Does anyone have an idea why AVAssets from a shared iCloud album cannot be exported via AVAssetExportSession? Experienced on both iOS 14.4 and some earlier iOS 14 versions.