Photos Having Trouble Accessing Files

I have a fairly large Photos library (35,000+ photos, 75+ GB), which was originally converted from an iPhoto library that I've been maintaining for many years. Many of the older photos are experiencing an odd problem, which is preventing syncing them to iCloud as well as exporting them from Photos.

When the problem manifests, the symptoms are such that…

• I double-click the photo to open it, and am presented with the message:

  Allow Photos to Access Your Media?

  Some of the photos and videos are stored outside of the Photos library. Photos needs permissions to access this media. Please choose the folder named "ddonahue" at the path "Macintosh HD/Users/".

• I click "OK" and am presented with a folder selection dialog, where I select my home directory as requested.

• I am presented with another dialog:

  Could not find the original file "dcpa_1457.jpg" in the selected folder. Would you like to try to locate it again?

(Or whatever the file name is for that particular photo.)

Naturally, the photo files are in the Photos Library in the home directory's Pictures folder. Buried in the package's folder structure. I can inspect the package contents and manually find the file.

I thought that perhaps I could manually drill down to the package contents folder which contains the file and specifically select that one. However, the folder selection dialog doesn't allow drilling into the Photos Library package contents. (The library file is greyed out.)

Is there some way to correct this? Even to include directly editing some library XML file(s) somewhere? (Which I could write code to automate, so that would be good given the volume of files.)