Unfortunately we were not able to resolve my issue and my problem has appeared to spread to Backup & Sync as well as FS.
MS Office: v16.24
Backup and Sync from Google: v3.43
I have tried full uninstalls (including preference and other Library files) of MS Office, Boxcryptor, Backup & Sync, and FS with no luck. Every time I try to open and save an encrypted Excel file, an error pops up and only the temp file is saved. I have also tried reverting to earlier versions of MS Office and earlier and experimental versions of Boxcryptor.
This issue does not affect my wife's mac, where (1) Google FS was never installed, and (2) MS Office: v16.16.