Hi there, @markh1.
Welcome to the Community. Allow me to help route you to the right support that'll ensure you can import your bank data in the QB Online Sandbox.
We have a specialized forum called Intuit Developer that can further isolate why you're getting the 404 error. I'd suggest posting this question there as they'll be the best support to assist you with this.
Here's the link for your reference: https://developer.intuit.com/.
You can visit us again if you have questions about other errors related to QuickBooks. I'll be here to help however I can.
