Hi there, @HPJ.

Thanks for joining us here in the Community. I'm here to help you get past the error code 8991 so you can back up your Data Protect successfully. 

Error 8991 usually occurs if QuickBooks detects an issue with a firewall or third-party security software, you will need to reconfigure the software and/or hardware to allow access to Intuit Data Protect files.

Since you've already done with performing the Diagnostic tool, let's go ahead and add Windows firewall ports so you'll be able to browse the internet and log in to secure websites, Intuit Data Protect should work.

Here's how:

1. Go to the Start menu, then type Control Panel in the search field.
2. Go to System and Security. Choose Windows Firewall, then select Advanced Settings.
3. On the left side of the Advanced Settings window, select Inbound Rules. Choose New Rule from the right side of the same window.
4. Enter the ports, then select Next.
5. Follow the prompts.
6. Follow steps 1-4 for Outbound Rules.

For more details in performing this procedure please refer to this article: Intuit Data Protect Backup Failed: Firewall or connection issue. 

I've includes some article that you can check out about IDP issues in QuickBooks: 

Intuit Data Protect backup incomplete. 

Troubleshoot Common Intuit Data Protect (IDP) Problems.

However, if the issue persists after trying these steps, I'd suggest reaching out to our Customer Care Team. They have the proper tools to check on your data and verify what's causing the issue.

To reach them: 

1. In your browser, navigate to https://help.quickbooks.intuit.com/en_US/contact.
2. Choose QuickBooks Desktop.
3. Click the issue or topic.
4. Click the Green Phone button to see the support number.

Please know that the Community has your back should you have any questions. Wishing you and your business continued success.