The "/usr/bin/postqueue -p" in Postfix > Module Config > System configuration should be changed to "/usr/sbin/postqueue -p"

After hitting the same problem which is described on https://www.virtualmin.com/node/21558, I've found out that number of our CentOS Virtualmin setups had "/usr/bin/postqueue -p" instead of "/usr/sbin/postqueue -p" in Postfix > Module Config > System configuration > Mail queue display command.