Mojave cannot open my Sierra photos library

Sorry guys but I need your expertise!

After my system crashed, I had to install Mojave, which I did not want. Now Mojave refuses to open my Sierra photos library. Any idea?

check for log messages when you launch the app.
It may show an error message.

@Christian Schmitz — Well the main error seems to be “Failed to open library Nouvelle Photothèque.photoslibrary Error: Error Code=328 “LiErrorLibraryTypeCheckFailedAllChecksFailed” UserInfo={NSURL=file:///Users/*****/Pictures/Nouvelle%20Photothe%CC%80que.photoslibrary/, NSLocalizedDescription=LiErrorLibraryTypeCheckFailedAllChecksFailed}”

do you use iCloud Sync?
Than maybe delete it and let it download photos again?

@Christian Schmitz — Thanks. That was actually the right move.

As a summary, and as a great example of counter-intuitiveness (pun intended):

  • For repairing a photos library:

    1. Hold Command + Option while starting up Photos
  • To re-make an entirely new local library from iCloud (only if the library has been saved in iCloud):

    1. Quit Photos
    2. Reopen Photos while pressing the Option key
    3. In the window to select a photos library, press the “New…” button
    4. Type in a new photos library name
    5. Go to Photos/Preferences
    6. In the General tab, click on the button “Use as system library”
      —> This will reload all the photos from iCloud, then recreate all the folders/albums. Do not expect it to be faster than 10000 photos/h