Google Apis sendVerificationCode returns backendError 39

Question

All of a sudden the sendVerificationCode auth (/identitytoolkit/v3/relyingparty/sendVerificationCode) POST call returns 503 backendError.

I couldn't find the error code in any of the Google API documentation as well. Appreciated any help. Thank you.

{
  "error": {
    "code": 503,
    "message": "Error code: 39",
    "errors": [
      {
        "message": "Error code: 39",
        "domain": "global",
        "reason": "backendError"
      }
    ]
  }
}