Hello there, @kerrih.
Thanks for posting in the Community. I’m here to lend a hand retrieving your customer invoices in QuickBooks Online.
Since this only happens to one of your customers, we need to perform some browser-related troubleshooting to fix the unusual behavior in the product.
Sometimes the cache stores up in your system and causes issues in your web pages like functions inside your QuickBooks not working. It’s possible that local internet files are behind this.
You can start by switching to a different browser or using a private window when logging in to QuickBooks.
When the program is not performing as it should and a private browsing session works, you need to clear your browser’s cache. Before doing so, make sure to take note of your saved passwords and URLs. Once done, here's an article with the steps on how to perform clear cache: Delete or disable cache and temporary internet files in your web browser.
That should do it. Let me know if this works out for you, and if I can be of additional help. Take care!