Slack chat integration broken after migration to Discourse Docker images

Continuing the discussion from Sidekiq not running. Sidekiq heartbeat test failed, restarting:

{
"error_key": null,
"request": "",
"response_code": "200",
"response_body": "{\"ok\":false,\"error\":\"not_in_channel\"}"
}

When sending test messages to other channels…
DiscourseChat::ProviderError

Some channels for this provider failed last time messages were sent. Click the error icon(s) to learn more.