Login into the Control Panel with the root user and its password and perform initial configuration. On the configuration step you can set new password for the admin user.
As workaround you can set desired password for the admin user using the /usr/local/psa/bin/init_conf utility. Then login into the Control Panel with the admin user using new password.
/usr/local/psa/bin/init_conf --set-admin-password -passwd NEW_PASSWORD