At times we often forget the webmin password for a given user, say root, or after repeated failed login attempts webmin locks the account and one cannot login anymore. In this case you need to wait for couple, three minutes and you will be able to try to login again. However, if you've forgotten your password and need to change it, then login to the server via ssh and run the following command:
/usr/libexec/webmin/changepass.pl /etc/webmin root NEWPASSWORD
/usr/share/webmin/changepass.pl /etc/webmin root NEWPASSWORD
Now go back to browser and try to login with the new password, it should work.