I would suspect issues with the server configuration with regards to the Ownership / Permissions:
http://help.websitebaker.org/pages/en/knowledge-base/files-and-permissions.phpFind out if your hoster allows to:
1) make an unpload via PHP
2) what permission is given to those files
3) where they are stored
You are not using Apple with MacOS as hardware, are you?
cheers
Klaus