Warning: dio_write(): cannot write data to file descriptor 10, No space left on device in /var/wkscript/common.php on line 393

i get a warning message show above.i can not upload new file to my domain via FTP as well as file management that zymic provide.
when i transfer via FTP,this error message found

450-0 Kbytes used (0%) - authorized: 6144000 Kb
Response: 450-Error during write to file: No space left on device
Response: 450 navi.php partially uploaded

my Disk Usage: 50 / 6000 mb ,still hv lot of space.What wrong?