redis-namespace | gem adds a Redis : : Namespace class which can be | Command Line Interface library

Looks like you're running into compatibility issues trying to use the latest version of the activerecord-import gem at the time of writing (released in October 2020) with activerecord 3.2.22.5 (released in September 2016). You do mention it's a rails 3.2.15 app but you're not using activerecord 3.2.15 which is confusing.

Try using activerecord-import 0.4.1 (released in July 2013) and activerecord 3.2.15 which should be compatible with rails 3.2.15.

Thanks to @Les Nightingill, I found the issue.

It was not directly linked to assets generation, but the probleme was indicated at the first error line in the logs :

Redis is not using HTTP protocol for client connection. So any http client like CURL cannot directly communicate with Redis server.

You can use redis-cli for testing

Credit for this answer goes to Jimmy Cuadra

The Lita handler you showed defines a route that will invoke an instance method called greeting_response when an incoming message matches. There is no instance method with that name defined, hence the "undefined method" exception. You have a method with this name defined in the other file, but a method defined at the top-level scope is not the same thing as an instance method in a class. If you want that method to be included in the Lita handler class, you should define it in a module that you include in the handler.

Redmine doesn't store any session data in its cache. Thus, configuring your two Redmines to use the same cache won't help.

By default Redmine stores the user sessions in a signed cookie sent to the user's browser without any server-local session storage. Since the session cookie is signed with a private key, you need to make sure that all installations using the same sessions also use the same application secret (and code and database).

Depending on how you have setup your Redmine, this secret is typically either stored in config/initializers/secret_token.rb or config/secrets.yml (relative to your Redmine installation directory). Make sure that you use the same secret here on both your Redmines.

Their docs make it look like you only need to set the URL for redis and not an instance of redis https://github.com/resque/resque/blob/master/README.markdown#configuration

It's a Kernel.warn call in redis-namespace.

You can disable warnings in Ruby with the -W0 command line option.

More info on disabling warnings here:
Suppress Ruby warnings when running specs

Cannot assign requested address - connect(2) for [::1]:6379 (Errno::EADDRNOTAVAIL)

Your rails application is trying to connect to redis via localhost but the service is on a different container. Try connecting to redis:6379 instead.

Okay for some reason, , group: :production was added to gem 'redis-namespace', group: :production in the Gemfile. Removing group :production fixed the issue for me