Based on the log provided, it is likely the proxy issue you identified is related. The onboarding application does not support this setup, as it prevents validation of the expected domain for the cloud side of the connection.
As a workaround, you may be able to configure your proxy settings to allow an exception to api.meraki.com on TCP 443 from the computer where you are onboarding so that the certificate provided from the API server is sent to your computer unchanged.
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