error while processing the authentication request

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Had a multisites, previously setup wp-mail-smtp successfully for 3 sites.
But while trying to add another 2 sites recently, got “error while processing the authentication request.” Meanwhile, no way to send a test email since the “Send Email” button is blocked.
Had verified the Google API credentials, all Authorized JavaScript origins and Authorized redirect URI are same as previous 3 sites.

Any suggestion where to look into, highly appreciate