Thanks a lot, @Falco! Your suggestion took me to the solution.
Indeed I was not following the installation procedure, more precisely I was not passing through the initial discourse setup (with the discourse-setup command).
I was just using a “launcher rebuild” command with the configuration file taken from the previous install.
Using discourse-setup allowed me to recreate the config file from scratch and then I modified it to add my custom paths and plugins, did a rebuild and everything worked as before.
Thanks again for your help!