I found the solution !
I had tried by importing credentials.yml in rasa-production/app, then I thought that rasa-production was connecting to the project on rasa-x (rasa-x/app/git/1/) when it is going to search in rasa-x/app/. I was misled by the fact that the stub code of the channels connector appears in the credentials.yml of the project and not in the general one.
To summarize, you have to implement the channels connectors in rasa-x/app/credentials.yml