Thank you for visiting the QuickBooks Community. I'll be sharing details on why you're receiving an error message about time out. Then, ensure you'll be able to remove this while working with QuickBooks.
QuickBooks Online has a timeout feature for your security to help prevent unauthorized persons from accessing your data when you're away from the computer.
When you're working in QBO and inactive for a period of time, you'll see Your session has a timed-out error message. To isolate this, I reccomend going to the Account and Settings page and making sure that the Sign me out if inactive for option is set up to 3 hrs since the default setting is 1 hour.
Go to the Gear icon ⚙️ and select Account and Settings.
Choose the Advanced tab.
Click the pencil icon ✎ from the Other preferences section.
Choose 2 or 3 hours from the Sign me out if inactive drop-down arrow.
Click Save and Done.
If the issue continues to occur, I recommend logging in to your QuickBooks account using a private browser as mentioned by Fiat Lux - ASIA. This is to rule out the possibility of a webpage issue, and private browsing doesn't store local files or cache.
Use these keyboard shortcuts based on the browser you're using:
For Firefox or Microsoft Edge, press Ctrl + Shift + P.