Let me help sort you out, @boberonicus.
I appreciate you for sharing the troubleshooting steps you've performed so you'll be able to copy your downloaded bank transactions into your register.
To isolate the error you've received, let's uninstall and reinstall your QuickBooks Desktop for Mac:
Once completed, you can try downloading your transactions again.
If the issue persists, I'd recommend contacting the dedicated Customer Care Team for QuickBooks Desktop Mac. A specialist will be able to further help you via a secured remote access session.
Here’s how you can contact us:
Fill me in if you have other questions about managing your downloaded transactions. I'm always here to help.