Alright, well then just undo the Webmin password thing and use your system password to login.
Edit the file /etc/webmin/miniserv.users, find the root line, and remove the password from that line and change it to x. It will look like a mess of letters and numbers, probably starting with something like $2$.
i.e. After this change, your root line should look like:
root:x::::::::0:0:::
Restart Webmin, and then login with your system root password. Stop all the nonsense with separate Webmin and system passwords; don’t bother with changepass, and never use the Webmin->Webmin Users module to set the password again.