On of the possiblility is: wrongly setting for temporary folder.
This may happen when we transfer file database from localhost to live server but we forgot to resetting the temporary folder..
So, we have change it corectly as below: -
Go to: http://SITENAME/admin/config/media/file-system
then change the temporary folder like: /sites/default/file/private/tmp
Another posibility is permission setting..look at it
* But first try to correct the temporary folder first..and if problem still persist..then proceed to the second option