"No space left on device" problem

Hi all,

I have a VPS with Ubuntu 14.04 and Virtualmin.

Today I got this error:

(I have the error in Italian… I try to translate it in English)

“Apertura di /etc/webmin/apache/site in fase di scrittura non riuscita: No space left on device”

“Failed to write to /etc/webmin/apache/site : No space left on device”

The problem is that I have a lot of free space on my disk. See the df result:

Filesystem 1K-blocks Used Available Use% Mounted on /dev/ploop13933p1 26702076 15113764 10435084 60% / none 4 0 4 0% /sys/fs/cgroup none 209716 112 209604 1% /run none 1048576 0 1048576 0% /dev/shm none 5120 4 5116 1% /run/lock none 1048576 0 1048576 0% /run/shm none 102400 0 102400 0% /run/user

Do you know what’s the problem? What can I do to solve it?

Thank you so much!
Andrea