the quota variable is not missing. If you use maildir, the complete quota script must be disabled. This is already fixed in the next ISPConfig release.
If you want to fix it in your installation, you must change the line
in the file /root/ispconfig/isp/conf/procmailrc.master
This will affect all new accounts. If you want to change existing accounts, you will have to edite the .procmailrc file in the home directories of the users or cahnge any setting of the user in ISPCOnfig and save the user.