I encountered this error as well. With some investigation, I found I just removed the docker container for redash scheduler with option ‘-f’ when deploy. After change the way, that stopping the container first and then removing it, and run ‘flushall’ in redis, the error went away.
Actually, the error is caused by the code in rq_scheduler/scheduler.py
scheduler_key = 'rq:scheduler'
if self.connection.exists(self.scheduler_key) and \
not self.connection.hexists(self.scheduler_key, 'death'):
raise ValueError("There's already an active RQ scheduler")