I'm here to help make sure you can reconcile your account in QuickBooks Online (QBO).
Before we start, may I know if you receive an error message after entering the ending balance and the date? Any additional information would help us provide the best resolution based on the error received.
In the meantime, let's log in to your QBO account using a private browser. It's possible that your browser stores frequently accessed data, thus causing QuickBooks to act weirdly. To do so, press this shortcut key on your keyboard to open a private browser:
Google Chrome: Ctrl + Shift + N
Mozilla Firefox: Ctrl + Shift + P
Safari: Command + Option + P
Once logged in, go back and reconcile your bank account again. If this works, it means that you need to clear the browser's cache so the system can start fresh. If you get the same result while using a private browser, I recommend switching to a different one.
After you have successfully reconciled the account, you may want to run the reconciliation report in the future. This will help fix your reconciliation if you have trouble reconciling the following month and when you meet with your accountant.
Please know that I'm just a reply away if you need further assistance reconciling accounts. Wishing you continued success, @max6121964.