I just encountered the exact same problem. The only causes I can think of were:
1. I deleted some invoice templates
2. I am accessing my account from another computer for the first time
This happens when your browser is full of frequently-accessed activities that cause you to get this error. You can use a private browsing window and press the corresponding keyboard shortcut:
Once done, if it works, go back to your previous browser and clear your browser cache so the system can start fresh.
Otherwise, you can try switching to a different browser.
I have included this article in case you have other questions about QuickBooks Online: Manage Customers And Income.
Please let me know if you have other questions. I'll make sure to help.