Finally, there was an issue with one config.json file stored in the remote repository used by the job.
Once that file was removed from the repo (as it was not required at all), the job started to work again.
I was able to locate the file using the --dev-debug parameter in the SF login command. In that way, the command returns the list of every action executed, so you can find where it is crashing.
Regards,
Javier Carvajal