If the following error is shown in the server logs when connecting via SSH with a keypair:
$ tail -f /var/log/secure
Dec 26 12:30:38 server sshd[3503454]: Authentication refused: bad ownership or modes for directory /home/user/.ssh
Check if the ssh configuration for the current user has the right access rights. Go in the user's home directory and do the following:
$ cd ~
$ chmod 700 .ssh
$ chmod 600 .ssh/authorized_keys
Connecting with a keypair should now be possible without a password promt.