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.
This error appears with every site and every reseller. According to statistics nothing is over quota. How to fix it? Thanks
I have no idea how the quota for the group admispconfig had been set. I have removed the limit and the problem are solved. This solves another problem also: http://www.howtoforge.com/forums/showthread.php?t=12578 Thanks Falko.
in my case i reached my hosting provider inodes limit (too many files in the cpanel hosting account). When deleted, error goes away.