ReadyNAS 6 stops working: kernel: fd_do_rw() write returned -28

Error -28 is no space left on device, but it is wrong:

LUN has 500 GB (of 3 TB) still free (chkdsk passed) and ReadyNAS:

Filesystem      Size  Used Avail Use% Mounted on
udev             10M  4.0K   10M   1% /dev
/dev/md0        4.0G  803M  2.9G  22% /
tmpfs           979M     0  979M   0% /dev/shm
tmpfs           979M  3.6M  975M   1% /run
tmpfs           490M  4.6M  485M   1% /run/lock
tmpfs           979M     0  979M   0% /sys/fs/cgroup
/dev/md127      5.5T  3.7T  1.8T  67% /data
/dev/md127      5.5T  3.7T  1.8T  67% /apps
/dev/md127      5.5T  3.7T  1.8T  67% /home

Could it be related to some BTRFS bug or reserved blocks?

http://logs.tvrrug.org.uk/logs/%23btrfs/2016-04-11.html#2016-04-11T07:25:23

root@NASNG:~# btrfs fi usage -T /data
Overall:
    Device size:                   5.44TiB
    Device allocated:              4.49TiB
    Device unallocated:          980.15GiB
    Device missing:                  0.00B
    Used:                          3.66TiB
    Free (estimated):              1.78TiB      (min: 1.30TiB)
    Data ratio:                       1.00
    Metadata ratio:                   2.00
    Global reserve:              512.00MiB      (used: 0.00B)

              Data    Metadata System
Id Path       single  DUP      DUP       Unallocated
-- ---------- ------- -------- --------- -----------
 1 /dev/md127 4.47TiB 13.00GiB  16.00MiB   980.15GiB
-- ---------- ------- -------- --------- -----------
   Total      4.47TiB  6.50GiB   8.00MiB   980.15GiB
   Used       3.65TiB  4.80GiB 608.00KiB

LUN size is 3.5Tb, 2.5Tb are currently used and probably 2.9Tb was the maximum used.

root@NASNG:/# ls -lh /data/veeambackups/.iscsi
total 2.9T
-rwxrwxrwx 1 guest guest 3.5T Oct 26 19:09 iscsi_lun_backing_store

root@NASNG:/# du -sh /data/*
799G    /data/Backup
2.8G    /data/Drivers
0       /data/home
304G    /data/ImagenDisco
0       /data/Pirovano
8.1M    /data/rsyslog
25G     /data/Software
2.9T    /data/veeambackups 

I copied 400 Gb (from 2.5 to 2.9Tb) and no problems so THERE IS free space.

any clue?