I want to make sure you're able to work in your QuickBooks Online account without any errors.
This particular error you've received can happen if you have more than one browser tab open with the same QBO account. To get this fixed, I recommend clearing the browser cache before you re-login to your account. The cache is for collecting frequently used data and can cause certain viewing and performance errors if it becomes too full. Clearing it will resolve this type of unusual behavior. Here's how to get this done in Chrome for example:
At the top right, click More (three vertical dots), then choose Delete browsing data.
Choose a time range, like Last hour or All time.
Select the types of information you want to remove.