Photo app will open one Photos Library on my external drive, but not the second Photos Library

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

How is the drive connected and formatted? You will get random errors, if the file system format is not compatible or the volume is not locally mounted. And it will not hurt to check the volume with Disk Utility. If a volume is getting old and starting to fail, it is not safe to use it any longer.

The easiest way to check the videos in the library would to downgrade the system back to Mojave, so you can browse the library in Photos 4. You need a backup you made while running Mojave to do that (Revert your Mac to a previous macOS version - Apple Support)

If you cannot revert to Mojave, you could try recreate your Photos Library from iCloud, if your library has been uploaded to iCloud Photos. Are you using iCloud Photos?