I'm here to help fix the issue you're having when updating your bank in QuickBooks Online (QBO).
There are times that the browser is already full of frequently accessed page resources (cache and cookies), causing some unusual responses. To isolate this, let's sign in to your QBO using a private browser (incognito).
If it's successful, return to your default browser and perform a clear cache to refresh the system. However, if the issue persists, try using other supported browsers.
Another option, utilize the trial version of MT Online to isolate the issue. It is designed to import into any bank or credit card in QuickBooks Online, so there is no specific setting needed. If the same error persists, there is an ongoing problem at your bank or connection.