Shared Album fails to add valid e-mail address

The inability to add a specific user's e-mail to a specific shared album persisted over multiple days, multiple devices (iPhone, iPad, Mac).

Symptoms again are specific:

1. Multiple shared folders and multiple subscribers per folder, 1000's of images per folder all working normally.
2. One specific e-mail address fails when added as a subscriber. Doesn't appear as an "invited". No notification e-mail sent.

Finally began working again after force quite of Photos application which failed to close properly on a rebooted Mac.

Unclear whether this actually had anything to do with the problem.

Best guess is that the e-mail was in a wedged state within the iCloud shared albums database.

The e-mail was previously a normal subscriber to the shared album.

When the user's iPad stopped showing new additions to the shared album,

removing them and re-adding them was attempted as a possible solution.

The re-adding failed repeatedly over a period of many weeks.

Yes.

I continued trying different ways to add them.

Restarting a Mac and starting Photos 4.0 logged in to the apple ID which owns the shared album initially failed to show anything under the "Shared" tree.

Quitting the Photos application displayed a "closing library" window with spinning animation which persisted for hours(?).

After returning to the Mac and force quitting Photos some time later, Photos restarted normally, displayed content under Shared after a longish but not unexpected delay; and finally was able to successfully add the problematic e-mail to the shared folder.