DBI connect failed : Access denied for user 'root'@'localhost' (using password: NO)

Try running this as root:

dpkg-reconfigure mysql-server-5.0

I installed mysql-server and mysql-client before installing Virtualmin 3.75.gpl. During mysql installation,

I had already set the mysql root user's password to XXXX

Then, later in the Virtualmin Post Installation Wizard, it asks for the password,

so type XXXX again.

Virtualmin then tries to set the mysql root password to XXXX

which naturally fails because the root password is already set to something. Hence, I also got the error message

DBI connect failed : Access denied for user 'root'@'localhost' (using password: NO).

Later I went to Webmin's mysql module configuration page and set the mysql root password to XXXX and everything worked fine afterwards.