You are saying the conversion completes but the file never opens?

Hello there, @euroswiss.

Thanks for reaching out to us. Allow me to help ensure you're able to open your QuickBooks Desktop Mac 2019 successfully. 

To isolate this issue, after you converted the file from the old version of QuickBooks Mac to the new one, you'll need to save the file to the documents first and restore it. Once done, you can now update the file. 

To restore attached documents, here's how: 

1. From the QuickBooks menu, select Preferences and click Attachments.
2. Click Attached Documents Library menu and choose Select Existing.
3. Select the Attached Documents Library folder.
4. Close the Preferences window. 

For additional reference, you can check these articles below: 

• Update QuickBooks Mac Desktop.
• Restore a backup (.dmg) in QuickBooks Mac.

I also recommend calling our QuickBooks Desktop Mac Support Team so they can do a remote session and assists you further with the steps. 

Here's how you can contact our phone support: 

1. Click this link: https://help.quickbooks.intuit.com/en_US/contact.
2. Select QuickBooks Mac Desktop.
3. Type-in Customer Care on the search bar.
4. And click Get the Phone number below. 

Please let me know how it goes. If you have any other concerns, don't hesitate to click the Reply button below. I'll be around to help