How to limit Airflow to run only one instance of a DAG run at a time?

You've put the 'max_active_runs': 1 into the default_args parameter and not into the correct spot.

max_active_runs is a constructor argument for a DAG and should not be put into the default_args dictionary.

Here is an example DAG that shows where you need to move it to:

dag_args = { 
    'owner': 'Owner',
    # 'max_active_runs': 1, # <--- Here is where you had it.
    'depends_on_past': False,
    'start_date': datetime(2018, 01, 1, 12, 00),
    'email_on_failure': False
}

sched = timedelta(hours=1)
dag = DAG(
          job_id, 
          default_args=dag_args, 
          schedule_interval=sched, 
          max_active_runs=1 # <---- Here is where it is supposed to be
      ) 

If the tasks that your dag is running are actually sub-dags then you may need to pass max_active_runs into the subdags too but not 100% sure on this.

I changed to put max_active_runs as an argument of DAG() instead of in default_arguments, and it worked.

Thanks SimonD for giving me the idea, though not directly pointing to it in your answer.