Set the disk quota higher for that user / domain!
You can do this in ISPconfig > ISP Manager > the-domain-with-the-problem > Space MB:
If you set "Space MB:" in the Basic tab to -1, it's unlimited.
If you want to set the disk quita per user, go to the "User & Email" tab, click on the user name, and set WebSpace MB and MailSpace MB to what you want it to be.
Never execute code written on a Friday or a Monday.