Warning: rmdir Directory not empty in _boost_rmdir

Hi great module. I got this error when I run cron not sure what it is all about

Warning: rmdir(cache/normal/www.spice4life.co.za//user/sites/default): Directory not empty in _boost_rmdir() (line 1090 of /usr/www/users/spicevfdfx/sites/all/modules/boost/boost.module).

Thank you for your time and effort in advance.