Photos failing to import: "The file doesn't exist"

As a test launch Photos with the Option key held down and create a new, test library. Try importing the photos again to see if the same problem persists. Does it?

If it doesn't you can do the following:

Starting Over From Scratch With a New Photos Library.

1 - launch Photos with the Option key held down and create a new library.

Already done.

2 - open your old library package with the Finder as shown in this screenshot:

3 - locate the Master folder in the old library and drag it onto the Photos icon in the Dock.

This will create a new library with the same Moments as the original library but will not include the metadata, albums, books slideshows and other projects.

Note: your current library will be left untouched for further attempts at a fix if so desired.

Once the import is complete verify that the new library has all of your photos and working properly before deleting the older, problem library.

It's worked for years, though, so I don't think there's a file system incompatibility.

But check the file system, just to be on the safe side. You can see the file system format In the Info panel, if you select the drive in the Finder and use the command "File > Get Info ⌘I"

The "General" section should be showing the format either as "MacOS Extended(Journaled)" or "APFS", and not "case-sensitive". Any other system will be problematic and you should not try to repair the library on a drive with an incompatible format. Then remove the library from that drive, before you try the repair.

Is your Photos library in the Pictures folder or on an external drive? You can get error messages like you are seeing, if the library is on an unsuitable external drive, for example, if the format of the file system on the drive is different from the format on the internal drive. Or the external drive is case sensitive and the internal drive is not. Or problems with the file ownership. It could also be an incomplete copy, if you keep the Photos library on a synced volume - Google drive, iCloud Drive, Dropbox - the syncing can mess up the internal folder structure. Or running CleanMyMac or similar can damage the library.

so I'll look into how to repair the library

Have you tried the repair tools in Photos? See: Repair your library when it has problems - Apple Support

I don't want to throw away almost a decade's worth of metadata, though,

Instead of using the naked image originals in the Master folder, you can try to export the originals with XMP sidecar files., if exporting is still working.

Test, if you can use "File > Export > Export unmodified originals" to export the images from your library. And enable "Export IPTC as XMP". This way your originals will be saved as

Sounds like the internal structure of your Photos Library has been damaged. ('2007115-022850' is almost the same as a folder name that might appear inside a Photos Library package. It should include one more digit. Possibly some index is damaged and contains a bogus folder name, or something removed a valid folder.)

You're welcome 🙂

I'm glad you could save your library.