Synology ssh passwordless: look at the permissions!

I bought a new Synology DS214SE after my beloved Maxtor Axis Central (with custom firmware) apparently has become too old to serve me as it should (a long story).
I tried to set up a passwordless, key-based authentication to the NAS, with no success.
I modified /etc/ssh/sshd_config
enabling what should be modified.
Then googled a while, then read the useful Zarino Zappia's post. No luck.

Anyway I eventually found a solution :

cd /var/services/homes/
chmod 755 admin

solved (as suggested here on superuser.com)