Openvz problem, it is confusing because it's an inode issue and there is enough free space.

cp: cannot create regular file `forums/memberlist.php': Disk quota exceeded
/dev/simfs             60G   20G   41G  33% /
none                  2.0G  4.0K  2.0G   1% /dev

cat /proc/vz/vzquota
qid: path            usage      softlimit      hardlimit       time     expire
26000: /vz/private/26000
  1k-blocks       20447708       62914560       62914560          0          0
     inodes         200001         200000         220000 1295689296         

Solution, increase inodes:

vzctl set 26000 --diskinodes 999999999 --save