401 Error with Gmail

One day the plugin just stopped working, but when I checked the setup settings, all was OK, the credentials and all. I tried to authorize the plugin using the “Allow plugin to send emails using your Gmail account” button, but the notification “There was an error while processing the authentication request. The nonce is invalid. Please try again.” pops-up everytime. I also tried deleting the last credentials and creating new ones, but doesn’t fix the problem at all.

Email Source: Contact Form 7
Mailer: Google / Gmail
{
  "error": {
    "code": 401,
    "message": "Request is missing required authentication credential. Expected OAuth 2 access token, login cookie or other valid authentication credential. See https://developers.google.com/identity/sign-in/web/devconsole-project.",
    "errors": [
      {
        "message": "Login Required.",
        "domain": "global",
        "reason": "required",
        "location": "Authorization",
        "locationType": "header"
      }
    ],
    "status": "UNAUTHENTICATED",
    "details": [
      {
        "@type": "type.googleapis.com/google.rpc.ErrorInfo",
        "reason": "CREDENTIALS_MISSING",
        "domain": "googleapis.com",
        "metadata": {
          "service": "gmail.googleapis.com",
          "method": "caribou.api.proto.MailboxService.SendMessage"
        }
      }
    ]
  }
}

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