Is there a password already set in this case? The Virtualmin installer is expecting that there isn’t a password set there… however, you can go into the MySQL module to tell it what the MySQL password is.
@andreychek; this error still exists to date… On a brand new centos 7 (digitalocean) I get the above error also. When I went to the mysql module, and entered the password that I set during post installation wizard, I get: “Login failed : Incorrect administration username or password”