You have a provisioned Heroku Redis instance, and credentials have changed.
Occasionally we will roll Heroku Redis credentials for several reasons. Normally this is around hardware failures but we also perform cred rolls for security reasons - typically this involves us setting the new configuration variable and restarting your application. This can cause issues if you have manually copy and pasted these credentials elsewhere, or have hardcoded them into your application.
We now let you attach our Heroku Redis addon to multiple apps as part of the platform and credential updates like this would also update the attached applications. See managing add-ons for more info,