Thank you for reaching out to the Community. Allow me to help share some updates about the Banking Error 9999.
The error 9999 is a reported script error that interrupts the connection between your bank and QuickBooks Online. Our engineers are currently investigating the occurrence of this error and are working on the fix of this unexpected behavior.
Also, I recommend reaching out to our Customer Care support. They have the tools that can add you to the affected user's list. This way, you'll receive an email directly from our engineers once updates are available, or once the issue is resolved.