I found this problem at the first time when webmin installed. Also this happens to everyone at one time or another. You go on vacation and when you come back, you forget all of the passwords on your computer.
I failed logging in several times and then Webmin blocked my IP. Getting localhost blocked isn’t a good thing to see. So I searched the Internet for a way to reset the password and I found this procedure:
1. Login to your computer as root. Type on browser address bar https://localhost:10000/
2. If you are running a Debian distribution (ubuntu), enter the following command:
/usr/share/webmin/changepass.pl /etc/webmin username password
3. Login to Webmin with your reset password.