I followed DigitalOcean\'s tutorials on how to set up your server with SSH, creating a new user, firewalls etc. The first tutorial in the series is linked under:
Please try to restart webmin with the following command and check if you are getting any error.
/etc/init.d/webmin restart
If it's working fine, Then defiantly there is an issues with the firewall OR some ports are blocked at your DC end. Please try to stop firewall and check again.