Hello there, @JLE59.
I appreciate you for letting us know the troubleshooting steps you've tried to resolve this error. Let me help you fix it by checking your bank connection and QuickBooks company file.
Before following the steps, please make sure to back up your company file. After that, create a test company file and connect it to your bank. By doing so, we can check if the error is caused by a problem with your bank, or damaged data in QuickBooks.
If you get the same error, it's possible there’s a problem with the connection from your bank. Since you recently upgrade to QuickBooks 2019, some banks need you to confirm your identity before you can use any of their online services.
Log in to your bank account on a browser, then check if you have any bank security notification. If there’s none, contact your bank directly to help you verify this.
If you don’t get any error, this could be caused by data damage. To resolve this, you can run verify and rebuild data on your company file. Then, ensure the online bank information is correct and reset the connection to your bank to refresh your banking services.
If you get the same result, let's check if there's an Accountant's copy restriction. If so, you may need to remove it to change your account customer ID. You can use the following article for the things you need to take note before removing it and for detailed steps: Remove the accountant's copy restriction.
Once done, you can refer to this article to update the customer ID in the Charts of Accounts or check the online banking credentials, and reconnect to your bank: Invalid Customer ID number.
Drop me a comment below if you have any other questions. I'll be happy to help you some more. Have a great day ahead.