There might be a problem with your external drive.
Can you tell us more about the external drive, thabo_otter?
- What is the file system format?
- How is the drive connected to your Mac?
- Has the drive been used by Time Machine or is some third-party backup software installed?
Photos cannot work with a library reliably, if the external drive is remotely mounted and not directly plugged, or has been used for Time Machine backups.
You may want to check the external volume with Disk Utility, if there are any hardware problems.
The error code "5416" in the range of QuickTime Streaming errors. It could be a problem with one of the videos in your Photos Library. Do you have older videos in the library? The upgrade to Catalina made some older video formats incompatible. We should weed out and convert older videos to a compatible format, before upgrading the Mac to Catalina.
If you have videos in your library, that can no longer be processed, they need to be removed.
This list can give you an idea which video formats to avoid: About incompatible media in iMovie for macOS - Apple Support