Issue
You have a provisioned Heroku Redis instance, and credentials have changed.
Resolution
Occasionally we will roll Heroku Redis credentials for several reasons. Normally this can happen if the Redis instance goes through maintenance (for example, in the case of hardware failures) but we also perform cred rolls for security reasons.
This can cause issues if you have manually copy and pasted these credentials elsewhere, or have hardcoded them into your application.
When credentials are rotated, Heroku will automatically update the configuration variables related to your Redis add-on (this is REDIS_URL, or any other corresponding config variable for the add-on attachment). This process will trigger a new release and will restart your application.