Mac App Store -can't purchase/re-download apps ("error 500")

[macstatic]

My wife just ran into a problem with her Mac where she suddently couldn't open an app ("Wallet" by Acrylic Software) previously purchased through the Mac App Store. There was an error messages saying something about it being damaged, and that we needed to delete it, then download it again from the App Store. She's running MacOS 10.14.6 Mojave.

Fine -a little frustrating and inconvenient to reinstall something that worked the other day, but no biggie except I wanted to ensure its data didn't get deleted along with it, so having the free AppCleaner installed, I made sure to make copies of the files/folders it would delete by default before actually going ahead with the delete (come to think of it I might have left out the deletion of some of its files even though I tried to make sure I had made backups).
We do have a Time Machine drive attached as well, but I try not to rely 100% on a single backup.

That said, after having deleted the app in question, emptied the trash and rebooted the computer I keep getting an error message whenever I try to re-download it from the App Store app ("The operation couldn't be completed (com.apple.commerce client error 500)"):

Following this I've looked up solutions for this and other Mac App Store related errors and tried just about everything, but still no go.
This is what I've tried so far:

1) deleted a bunch of App Store related preference files in the ~/Library/Preferences/ folder (as explained here)

2) deleted a couple of App Store related cache files in the /var/db/crls/ folder (as explained here)

3) updated the payment information for her Apple ID

4) logged out and in again of her Apple ID (in multiple places: the Mac App Store app, the "iCloud" system preference, and iTunes)

5) changed the Apple ID password (by going to the Apple account website), then logged into the various locations mentioned in step 4

6) cleared a different cache (from the one in step 2), by deleting everything inside the ~/Library/Caches/com.apple.appstore/ folder (as explained here)

7) opened the following folder: /private/var/folders/ and looked for a sub-folder named com.apple.appstore, which I didn't find (as explained in the same link as in step 6)

8) Booted the Mac into Safe Mode (as explained here)

9) changed the Kaychain access setting (as described here)

10) changed the App Store app settings via the Terminal app using this command:
defaults write com.apple.appstore.commerce Storefront -string "$(defaults read com.apple.appstore.commerce Storefront | sed s/,8/,13/)"
(as described here, in the same link as in step 9)

11) emptied the App Store app's temporary cache folder by issuing this Terminal command:
open $TMPDIR../C/com.apple.appstore/
(as explained in the same link as in step 9 and 10)

12) Tried re-downloading a different app that had been previously purchased/downloaded for free from the same Apple ID. This resulted in the same "error 500" message as in my screenshot above

13) Tried downloading a new app (not previously purchased before) -a free one, using the same Apple ID. This also resulted in the same "error 500" message.

14) rebooting, shutting down/powering up the Mac, logging out of the user-account, resetting the PRAM.

15) de-authorizing the Mac (from within iTunes), then re-authorizing it

That's about it. What else can I try?

 

[Eriamjh1138@DAN]

Try downloading with her ID onto another Mac or a different user account in macOS on the same Mac.

 

[macstatic]

Try downloading with her ID onto another Mac or a different user account in macOS on the same Mac.

Good suggestion!
I think I tried that as well (on a different MacOS login account on that same Mac), only to get the same "error 500" message as with her regular login account.
But I'll give it another go anyway (in case I forgot to try it out/am confused about the result) and see what happens.

On my own Mac I run the same OS (Mojave 10.14.6) and have a couple of additional MacOS login accounts
-can I log into one of those users, then use her Apple ID username/password in the App Store app to try to download the mentioned app (or another app), or will that confuse/mess up things since I use my own Apple ID on the physically same Mac (although on a different user)?
And do I have to ensure that I don't download an app that I already have myself, on the same computer? I assume both "my apps" and "her apps" (owned by different Apple IDs) will all go into the same "Applications" folder since it's the same computer?

 

[HDFan]

What does Apple Support say?

 

[macstatic]

I haven't found out what Apple says about the subject of using different Apple IDs on the same computer, and how that would possibly mess up things.
I actually tried to log into another MacOS user-account on my wife's Mac, then open the Mac App store app and log in with her Apple ID to download the app (and other apps), but got the same "error 500" message as from her normal login account, so apparently something's wrong at a computer level, and not just account level.

But... I did try something else that at least seemed to resolve the problem of that broken Wallet app that I couldn't get to re-install:
Using my own Mac Pro I decided NOT to try and install anything on my daily setup (MacOS 10.14.6 Mojave) in case it messes things up, but on a physically separate drive where I have other MacOS 10.15 for testing things out.
So once having booted into MacOS 10.15 I logged into both the iCloud system preference and the Mac App Store app using my wife's Apple ID. Is it correct to assume that, as far as Apple sees it, that particular setup basically means it another computer belonging to my wife?

So within that scenario I re-downloaded her previously purchased Wallet app, which I also tried out on that computer. Worked fine!
Next I copied the Wallet app from my Mac's Applications folder over to her physical Mac's Applications folder (using a home network).

Back on my wife's Mac I double-clicked that Wallet app and (as far as I remember) I was first asked to enter my wife's Apple ID and password, followed by it running normally. Once that was done I restored its data from her Time Machine backup.
So in short.... I downloaded the app to a different computer, but via her own Apple ID, then transferred over to her own physical Mac. Won't that mean this is essentially her app (just as if she would download it from the Mac App Store app on her own Mac)?

I hope that solved the most crucial problem (not being able to use her Wallet app), but I still haven't solved the Mac App Store app issues.