I've reauthorised the connection with Nationwide many times, but it keeps coming up with an error on the Quickbooks side (390), just saying try again in a few minutes?

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This error typically doesn't last more than 48 hours and should be resolved on it's own. If the error persists, you can try disconnecting and reconnecting your bank account in QuickBooks. 

Be sure to check that you're using the correct login credentials (username and password) for your bank account and complete all necessary authentication steps if your bank requires multi-factor authentication (MFA).