desktop app

I have the steps you can perform to fix the error when opening QuickBooks through the desktop app, @NTH1. 

Based on the error message you've mentioned above, it looks like you're using the QuickBooks Online (QBO) desktop app. You're on the right track in cancelling and relaunching it. Since this doesn't work, the app data may be full. I'd suggest clearing its cache to restore the application to its default settings. Doing so will help you fix the error. I'll guide you how. 

1. Go to Help from the top menu. 
2. Select Reset App Data.
3. Allow a moment for the process to complete, and then close the app.
4. Be sure to right-click the app and choose Run as Administrator when re-opening it.

The screenshot below shows you the first two steps. 

Once done, you can continue performing your essential tasks in the program, such as creating invoices. 

Furthermore, I recommend visiting this website: QuickBooks App for Windows and Mac. It provides you more details about opening a new window, transaction, and report from the top menu. It also contains links to speed through common workflows using keyboard shortcuts, multiple windows, and taskbar preview. 

Please know that I'm here anytime you have other concerns. Stay healthy and keep safe always, @NTH1.