Most of the time, error 400 means that the server is unable to process the request being sent due to an issue that is perceived by the server (corrupted). If you're trying to connect your bank, we can perform some steps to isolate the problem. First, log in to your bank's website to check if you're getting the same error. If the issue persists, the error may be coming from your bank. You may reach out to them to investigate this further.
On the other hand, you can try manually updating your bank account to refresh the connection between QuickBooks and your financial institution.
Go to the Banking tab.
Select the blue tile at the top for the bank account you want to update.
Click the Update button. If you see a message about additional authentication, follow the on-screen instructions. Not all banks require this, but some do for added peace of mind.
I'm also adding this article for your reference about fixing banking errors in QuickBooks Online.
If there's anything else you need help with, let me know by adding a comment. I'm always here for you. Have a good day!