Unknown error exporting videos from iPhoto Mac to Untitled folder

As Matti Haveri indicates, going into the Photos package is the Nuclear Option, and you should only do that with a copy of the Library, as you may well make that copy unreadable. It looks like the database may have lost connection with the original movie files. Do you have these originals some place? As with Matti, I keep all originals in Finder Folders on external drives as one form of backup.

But first,

• Have you re-started your Mac?
• Have you re-started your Mac in Safe Mode?

           Start up your Mac in safe mode - Apple Support 

• If this is a Mac, where does your Photos Library reside-- in the Pictures folder of your internal drive, or on an external drive? or on a network? on iCloud drive? (As Yer_Man already asked…)
• Are you using iCloud Photos to synchronize your devices?
• Do you have “Optimize Storage” checked in Photos’ Settings
• If you use iCloud, what do you see at iCloud.com? Do the videos work there?
• Do you have more than 10% free storage on your iCloud account? (if applicable)
• Do you have more than 10% free storage on your Mac?
• Were you ever able to play, edit, or export these videos in Photos?
• Did these videos come from an iPhone or from some other source?

In Photos' Settings>General

Do you have "Copy Items" checked? Could it have been un-checked at some point?

Respectyourclients wrote:

I open to tips.

Maybe you could by brute force copy the offending movies from the Photos library? I guess this does not work but is worth a try:

In Photos do a search to find the movie you can't export, and then drag that Photos library movie thumbnail to the Applications/Utilities/Terminal.app window.

If you also then see the "Unknown error" I save you from the details how to find the original movie inside Photos library because it seems the connection to the original movie has been lost in the database, or maybe the original movie is not there at all.

I guess then the only way to try to find those offending movies might be to make a copy or an extra backup of the Photos library (manually messing and making changes inside the library might further corrupt it so it is best to do that to a copy!!) and try to find if there is anything interesting to salvage from the obscurely named images and movies (F0928C00-633E-42A4-BB3D-2001E36F017C.mov etc) among the originals. So in Finder control-click the copy of the Photos library and choose "Show Package Contents" and then navigate to .../originals/... and inspect its contents, and copy anything interesting to another folder.

If all else fails and others do not chime in for anything better, I'd do a File > Export > Originals, and make a brand new Photos library from them (although personally I archive originals in plain Finder folders on external backups and use Photos.app only as a viewer).

Can you view the images at full size in Photos? Can you edit them?

Where is your library stored? If on an external what format is that?