Quicken Mac: cannot write to the file. "You do not have permission"

Mac 10.15.7 (Catalina)
Quicken 6.8.3 (Mac / Canada)
---
My wife and I each have our own user accounts on the Mac. We share a Quicken file. Rather than having her log into my user account (a nuisance for her when she's doing banking in her user account), I placed our Quicken data file into /Users/Shared.
I then switched to her user account and went to Quicken to open the data file (I had closed it on my side). Quicken said 'Quicken cannot write to your data file. You do not have permission to write to the file. Change your allowed permissions to read/write for the data file, then try again to open it again.'
So I did exactly that…switched to my account, found my data file, Get Info, added my wife's user account to 'Sharing and Permissions', set her account to Read/Write.
Went back to my wife's user account…same message. Get Info window shows her account as having read/write privileges. Then I switched back to my account…and now my account says the same thing.
So I moved the data file back into my Documents folder, and I still get the same message. Quicken reports that I do not have permissions.
Now I can't get into my data file at all. From any account. Help!