This is so stupid but took my 1 hour to figure out.
I installed Open Media Vault to create some raids and home NAS on a debian machine. However, after the installation it started to reject my ssh connection to the machine.
Things to look for:
sudo vim /etc/ssh/sshd_config
# Add this line: AllowUsers your_username
# Change this line: PermitRootLogin yes
# Restart the service.
sudo service ssh restart
# Mine is not fixed so I had to put my username into ssh group
sudo usermod -aG ssh your_username