I have a troubleshooting step to share that can resolve the error, Benjamin.

Are you getting an error message that says "Your file is too big to export" when converting your file? The file can only have a maximum of 350,000 targets in order to import.

You might want to condense your data before converting the file. Follow the steps from this article on how to do that: Use the Condense Data utility.

You can also check out this article if you're getting a different error: How to fix errors when converting from QuickBooks Desktop to QuickBooks Online.

Please add a reply below if you have follow-up questions. I'll get back here to answer them as soon as I can. 

Hello, @Ben-Jamin.

Thanks for providing the exact error message you encounter.

Does this happen when you try to upgrade the file to QuickBooks Desktop 2019 or only when you convert the file to QuickBooks Online? 

Here’s an article that can help upgrade or convert your company file to a newer version of QuickBooks:  https://quickbooks.intuit.com/community/Getting-Started/Upgrade-or-convert-your-company-file-to-a-ne....

If you continue to get the same result, I’d suggest getting in touch with our Technical Support Team and have them process the upgrade for you. Here’s how you can reach them: 
1.    Select your QuickBooks Product.
2.    Choose your QuickBooks version.
3.    On the Contact Us page, click a topic.
4.    Click on the Get Phone Number button to see the support number.

Keep me posted on how the call goes and if you have additional questions. I’m always here to help. Thanks for dropping by and take care!