Hi, thank you for your answer. What I had missed was to define the device type on the client, so there was no match.
Now we could download a single file successfully, using the demo server. By the way, it can only send the domain s3.docker.mender.io, we could not change it and that could be nice, given that clients and servers live on different networks. The only workaround we found was to resolve that domain through the client´s /etc/hosts file, which is somewhat dirty.