As @marc mentions, I believe the first thing to do is to check for the cause of error in /var/log/auth.log.

Just to bring up my case, which might not be so uncommon, I got a "bad ownership or modes" error.

...
... sshd[14396]: Authentication refused: bad ownership or modes for file /home/foo/.ssh/authorized_keys

which immediately brought me to the issue: the file was editable by group and others.

Indeed, as recommended in the manual page of ssh:

    ~/.ssh/authorized_keys
         Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be used for logging
         in as this user.  The format of this file is described above.  The content of
         the file is not highly sensitive, but the recommended permissions are
         read/write for the user, and not accessible by others.

(A chmod 644 did worked for me)

As @marc mentions, I believe the first thing to do is to check for the cause of error in /var/log/auth.log.

Just to bring up my case, which might not be so uncommon, I got a "bad ownership or modes" error.

...
... sshd[14396]: Authentication refused: bad ownership or modes for file /home/foo/.ssh/authorized_keys

which immediately brought me to the issue: the file was editable by group and others.

Indeed, as recommended in the manual page of ssh:

    ~/.ssh/authorized_keys
         Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be used for logging
         in as this user.  The format of this file is described above.  The content of
         the file is not highly sensitive, but the recommended permissions are
         read/write for the user, and not accessible by others.

(A chmod 644 did worked for me)