Unable to import .abbu file into Contacts on Mac OS 15.1

Any suggestions how I can open egain access to my contacts?

A few weeks before I updated my MacBook Pro to Catalina (OS 15.0), I made backed up my Contacts by exporting them as an .abbu file. I then opened the file and verified the backup process worked.

Now running 15.1, when I go to Contacts > File > Import and select that file (as Apple instructs), no contacts import—just the name of my sole smart group. Same happens when I double click the .abbu file. The situation also occurs when I used a version of the .abbu file backed up a month ago via Time Machine.

(Also, I don't use sync them to iCloud or another device, so there's no device from which I can sync them over.)

The .abbu file is 229 MB too, so it clearly isn't empty.

Ctrl-clicking and choosing Show package contents opens the folder at the end of this post (the Images folder is empty and Meta has one .abcdg file—but Sources has three subfolders, each of which has a file structure similar to the the main folder's).

Any ideas how I can use those files to restore my contacts? Thanks!