There are 2 possibilities in this.

Either you have exceeded disk space or second reach max limit of inodes.

You can check first case using command df -h if you see free spae is available then its a clear case of Inodes exceeding.

df -i  will show this.

You need to increase the inode limit from hardware node using command

vzctl set 123 –diskinodes $(( 200000*3 )):$(( 220000*3 )) –save

You are done now.