Hi there, ronan-stoltz.
Welcome to QuickBooks Community!
Mobile applications save cache which causes issues. Go to your phone's Settings, select Apps, find the QuickBooks Online app and you'll see the option to clear it. Once done, log back in and see if it works.
Also, we need to make sure that it is updated. Visit Google Play or App Store to install the latest release. If the same thing happens, it's best to uninstall, then reinstall it.
Please check these articles for more details:
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