I got a solution to this problem, the .ftpquota file in /var/www/test.com was not up-to-date. I ran this command to fix it :
pure-quotacheck -u web1 -d /var/www/test.com
I have looked in ISP Config cron daily but I didn't see it...
I suppose I have to add a line in my crontab to update ftpquota files on a regular basis, is that alright ?
Thank you for your support.