Hi there, melany1.

Thanks for posting in the Community.

I've checked on my resources and found out that there is no issue with others having a problem to open their overdue invoices on their mobile apps.

To isolate the issue on your iPhone, I'd recommend uninstalling then reinstall it again. After reinstalling the app, you can now log back into your account and try to open the overdue invoices. You can refer this article for more detailed steps on downloading the QuickBooks Online mobile app. 

If persists, I'd suggest switching to a different browser or using a private window on your device. This way we can know if this is a browser-related issue. See the steps below on how:

Safari:

1. Open Safari on your iPhone.
2. Tap the Pages icon.
3. Choose Private.

Chrome:

1. Open Chrome.
2. Tap the Chrome menu button at the upper right-hand corner of the app.
3. Choose the New Incognito tab option from the menu.

Mozilla Firefox:

1. Open Firefox.
2. Tap the Tab icon at the bottom of the screen.
3. At the top of the screen, click the Mask button to switch to private browsing.

For future reference, you can check this article for more troubleshooting steps on different issues of the mobile app: Mobile applications hub.

Fill me in if you have any other concerns. I'll be more than happy to help you. Take care!