Cloudmin isn't using SSH key when choosing "Using SSH key" (it keeps trying to log in using password) while trying to add a system.
From the remote server logs: sshd: Failed password for root from 184.108.40.206 port 53403 ssh2
Details: - Cloudmin and Ubuntu are fully up to date - The remote system is an EC2 instance - Root user is passwordless
Additionnal details: - Selecting "Do not attempt SSH login" will successfully add the system. - Using a terminal with the key will work fine