After updating my home server to the latest Ubuntu release (Natty / 11.04) I found almost everything worked properly apart from Webmin refused to let me login. After some investigation, I found that my normal user was deleted. I reset the password via SSH using an included script.
Here’s the command:
$ sudo /usr/share/webmin/changepass.pl /etc/webmin root new_password
Then, logging in as root user to Webmin I could configure my normal user.