You probably set the root password in the Webmin Users module instead of using the system password…now you have two root passwords (a system password and a different one for Webmin). Don’t do that if you don’t want to keep up with two passwords. Use system passwords.
You can unset the root password by editing /etc/webmin/miniserv.users and changing the password for root from whatever it is (it’ll be a hash) to x. Restart the webmin service with systemctl restart webmin
Oh, wait. What the heck. I just read you renamed root? Why on earth?
So…the answer is login with your other username, right? (But, also, nobody do this. It doesn’t even make sense. Use either root or your default admin user, e.g. the first user account you created on Ubuntu, for Webmin.)
You can also create a normal system user with sudoALL=(ALL) ALL privileges, and it will be able to login to Webmin with root level access.
If you just renamed Webmin user (using Webmin Users module), it means there would be no correspondent UNIX user, that could be used with default authentication method, which is set to Unix authentication by default (for default root user) and which you most likely didn’t change to Webmin password authentication by selecting Set to… from the drop down menu: