Getting "botocore.exceptions.EndpointConnectionError" on lambda client using boto

I have a lambda function which invokes another lambda function using boto and it used to work. suddenly it started giving connection error. Below is the code of invocation-

lambda_client = s.create_client('lambda',
                                config=Config(connect_timeout=5, read_timeout=900, retries={'max_attempts': 2}))

def some_func(payload):
    response = lambda_client.invoke(
        FunctionName=os.environ['another_func'],
        InvocationType='RequestResponse',
        Payload=json.dumps(payload)
    )
    return response

it gives error - botocore.exceptions.EndpointConnectionError: Could not connect to the endpoint URL: "https://lambda.us-west-1.amazonaws.com/2015-03-31/functions/arn%3Aaws%3Alambda%3Aus-west-1%3A{account_id}%3Afunction%3Afunc_name/invocations"