Can we confirm what level of user access you have with QuickBooks Online.Is it custom/standard, company admin or master admin?
The app has recently been updated to remove a bug that was allowing custom/standard users to use it and nowonly company admin and master admin users can use the app.
Let's change the data connection of your device. If you're connected to Wifi, try switching to cellular data or vice versa. This way, we can check if this issue is due to slow data speed.