Disk quota exceeded problem

The problem in my case was I had exceeded the number of inodes (file entries) supported by the file system. The command

df -i

revealed the problem in my VM:

Filesystem      Inodes   IUsed IFree IUse% Mounted on
/dev/vzfs      1500000 1500000     0  100% /

Deleting some unnecessary small files or increasing the amount of inodes (more complicated) resolves the issue. A command like this can help locate which directories have a lot of files in them (in this case searching underneath /var):

find /var -type d -size +256k

The disk isn't full, but the disk space allowed for this user is full. You need to check quota(1), perhaps persuade the suspect to clean up their junk, or in an outburst of kindness increase it with edquota(8).