Exporting photo's in Sequoia 15.6 from Apple Photo's to a different folder.

The reason we ask about your using an external drive is that Photos Libraries run from an external drive that's not formatted for Mac in either APFS format or Mac OS Extended (Journaled) format become corrupted. Since Sequoia is intended to prevent Photos from even opening improperly formatted drives, it's unlikely the cause of your problems. But it's certainly worth asking.

This export problem seems unique-- I've not heard of it happening before. To narrow things down, try these steps:

• Restart the computer (of course)
• R e-start in Safe Mode. This bypasses certain potentially disruptive processes, and it often helps. Safe Mode is different for different computers, so see this: 

           Start up your Mac in safe mode - Apple Support 

• Try using another library if you have one. Or you can make a small test Library with a few pictures in it and see if the same thing happens. You can create a new Library by closing Photos and then option-clicking on the Photos icon.
• Make a new User and see if the same problem recurs with it. This is a bit harder, because a new user can't access a different user's stuff. You would need to move your Library out of your own user's folder up to the general "Users" folder. Or, you can just make a new Library for this user and import pictures to it.
• If using a different Library fixes the problem, then you should try to rebuild your original Photos Library--close Photos and re-launch by option-command-clicking the app icon. I'm suggesting this last because it may take a long time for iCloud Photos to re-sync with a rebuilt Library and for things to get back to normal.

The idea is to find out if the problem is with the Library, with your account, or with caches and login items, and stuff like that. One of these steps may fix the problem, but their failure to work also helps figure out what's happening.

Let us know what happens…

Perhaps the edited versions are corrupted in some way or missing or cannot be found.

Typically, this problem can occur after running a cleaning application on the Mac, like CleanMyMac, or similar.

Have the NEF files been edited in Photos or using an external editor, that is writing the edited versions back to the Photos Library? Or have the photos in question been migrated to Photos from iPhoto or Aperture? Any photos or videos migrated from Aperture or iPhone have to be reverted to the original before we can edit them.

You did not yet reply to Richards question, where your Photos Library is stored. Even when using iCloud Photos you are having a system photos library. Is this library in your Pictures folder or on an external drive?

As a test, if your library in iCloud is showing the same corruption, you may want to open your iCloud web page at www.icloud.com and try to download the edited versions from the Photos.app there. Can you download the edited versions directly from iCloud or have they not been uploaded to iCloud as well?

This isn't very clear, and you didn't provide much information. Just saying "not very nice" doesn't help much-- details are important.

Are you saying that using "File>Export>Export nn Photos" causes a crash? Where? When you first get the dialog or what? What error message? Are you meaning that "File>Export>Export Unmodified Originals" works and doesn't crash?

Cold you tell us:

• Are you using iCloud Photos to synchronize your devices?
• Do you have “Optimize Storage” checked in Photos’ Settings
• Where does your Mac's Photos Library reside-- in the Pictures folder of your internal drive, or on an external drive? or on a network? on iCloud drive?
• Do you have more than 10% free storage on your Mac?
• Has this problem always existed, or did it just start? Can you think of anything that changed that might affect your device?