Mail traffic empty
We have realized that the mail traffic is empty in ISPConfig 3.1.1p1.
As I analyzed the code in 100-mailbox_stats.inc.php, found the bug.
There are local variables and functions (e.g $mail_boxes), and member variables (e.g. $this->mail_boxes) as well. Some lines using the local one, and some lines are using the member variable / function.
I think you should delete the local variables and functions, and be sure using the class members.