monitor_tools.inc.php uses /var/vmail and not the mail directory specified in server config
The file /usr/local/ispconfig/server/lib/classes/monitor_tools.inc.php uses fixed functions on dir /var/vmail. If in the config another directory is specified for the mailfolders, it doesn't work anymore. It occurs 4 times.
function monitorEmailQuota() should be changed. The /var/vmail shouldn't be used, but the mail Homedir Path from the server config.