Redis raises "NOAUTH Authentication required" error but there is no password setting

I got error NOAUTH Authentication required when I connect to Redis server via command: redis-cli and run ping to check if Redis is working.

I found answer for NOAUTH Authentication required error which describes that this error only happens when Redis is set a password, but I checked Redis config file at etc/redis/redis.conf and there is no password setting.

Does anyone know that if there are other settings which can cause this error? Thanks for any help.

p/s: I am using Ruby on Rails web framework, Redis database is used for Sidekiq.

Edited: Redis version is 2.8.4. Server is hosted on AWS.

Currently, I decided to set a password for Redis server so that it can not be set password when it is running.

(When Redis server is restarted, it will work normal. You can run sudo service redis-server restart to restart Redis server.)