I'm here to help ensure you can see your clients and old invoices in QuickBooks Online (QBO).
There are instances that the browser is already full of frequently accessed page resources (cache and cookies), causing some unusual responses. Let's try signing in to your QBO account using a private browser (incognito).
Here's how:
Press Ctrl + Shift + N (Google Chrome).
Ctrl + Shift + P (Firefox).
Control + Option + P (Safari).
Ctrl + Shift + P (Microsoft Edge)
Once logged in, go to the Sales menu on the left pane to check and check if you can now see your clients' list and old invoices.
If they're now showing, return to your default browser and perform a clear cache to refresh the system. However, if the issue persists, try using other supported browsers.
For more information about managing clients and invoices in QBO, check out these articles: