Hi aileen1,

This happens when your browser is full of frequently-accessed page that cause you to get this error. You can use a private browsing window and press the corresponding keyboard shortcut:

Chrome: Ctrl + Shift + N
Internet Explorer: Ctrl + Shift + P
Firefox: Ctrl + Shift + P
Safari: Command + Shift + N

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.

Keep in touch if you have other questions. I'll make sure to help.