Hi there, @mark189.
Thank you for raising your concern here with us on the Community page. I'm here to ensure you'll be able to get back to business and open the app seamlessly.
As an initial step, I recommend clearing the app data to optimize its performance. Sometimes, the stored cache can cause unusual behavior with the system.
In the QuickBooks Online app:
In your device Settings:
Once completed, reopen the application.
In case you're getting the same error in the future, you can also uninstall and reinstall the QuickBooks Online app. This way, you can refresh the connection between your app and your phone.
To learn more about some quick steps to fix most app-related issues, you can open this article: Troubleshooting Quick-Start Guide for the QuickBooks Online app.
While working with QuickBooks, you can as well check out some related topics from our help articles as additional resources for additional tips and resources.
I'm more than happy to lend a helping hand if you have any other QuickBooks questions. Please feel free to add a comment below. Have a great day!