Thanks for sharing your concern here in the Community, @1st Choice.

I'm here to help ensure that you'll be able to reconcile your accounts successfully in QuickBooks Online.

The error that you received when reconciling your Amex bank transactions can be caused by the stored data on your browser. To fix this, I recommend performing the troubleshooting steps shared by my peer Charies_M above.

However, if the problem continues, I suggest reaching out to our Customer Care Team. This way, they'll be able to investigate the root cause of this matter and apply a fix.

To do that:

1. Click on Help at the top menu bar.
2. Hit on the Contact Us button.
3. Enter a brief description of the issue in the What can we help you with? box.
4. Press on Let's talk.
5. Select on Get a callback.
6. Key in your contact details, then tap on Confirm my call.

Once fixed, you can always utilize this link, which provides details about reconciling your accounts smoothly in the system: Learn the reconcile workflow in QuickBooks.

Please keep me posted on how things turn out. I'll make sure you're all set. Have a lovely day!