Thanks for bringing this to our attention, renciaminnaar.
This Unexpected error is often caused by a large number of cache piled up on you browser. I suggest performing some basic browser trouble shooting steps.
The first step to isolate this is to access your account through a private browser or incognito window. This is a good place to start the troubleshoot since this type of browser doesn't store cache.
You can refer to the following keyboard shortcuts below:
If this works, go back to your original browser and clear the cache to make sure your browser functions more efficiently. Otherwise, you can try other supported browser to help narrow down the cause of this behavior.
Here's a guide for instructions on how to fix common issues when reconciling: Fix beginning balance issues if you've reconciled the account in the past QuickBooks Online.
In addition to that, you visit these links about reconciliation in QBO:
Feel free to drop me a reply below if there's anything I can help you with. Wishing you all the best!
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:
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!