Check User's Shell: Ensure that the user "default" has a valid shell. You can check the user's entry in /etc/passwd
Good suggestion, imho, but you did imply you'd logged in as the user, so this is also (probably) a red herring.

Home Directory: Ensure the user "default" has a home directory. 

SSH Key/Password: If you're trying to log in using SSH keys, ensure that the public key is correctly placed in /home/default/.ssh/authorized_keys and that the permissions are set correctly
- since you've not logged in via SSH, this is probably also a herring.

However, the suggestion: If you’re using password authentication, make sure the user has a password set  Might be helpful.
I'm skipping firewall suggestion.
Log Files: Since /var/log/faillog is empty, also check /var/log/auth.log for any messages related to SSH login attempts 
- very good idea.

Three's more, but I'm guessing the log files are the most likely place to start, after the suggestions everyone else gave.