Issue
You're seeing the error ERR max number of clients reached and running out of connections in your Heroku Redis instance.
Resolution
There are several things you can do to avoid this, such as using a connection pooler to reduce the number of connections or setting an appropriate connection timeout to close inappropriate idle connections.
The following blog post goes through the above recommendations on connection timeouts and connection pooling, and includes other useful tips:
https://blog.heroku.com/real-world-redis-tips