Nevermind. I found the problem. There was a change of IPs for server and client a month ago, and a "rechange" a few days ago. Added the IP to /etc/hosts and recreated the public/private key exchanges, and all is fine now.
For those who are always prompted for a password, try this
ssh -l root clientIP whoami
ssh -l root clientHostName whoami
clientIP: e.g 188.8.131.52
clientHostname: e.g. mediaserver
One of the two should return
while the other returns
The authenticity of host 'files (184.108.40.206)' can't be established.
RSA key fingerprint is 67:3f:82:e4:1c:6a:71:b8:46:f3:8f:a0:28:c6:f2:25.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'mediaserver' (RSA) to the list of known hosts.
After that, run your backup and it should work just mine.