When connecting through a Microsoft ISA 2004 server, the FTP client has read only access to the files and folders. e.g. Uploading, renaming or deleting a file or folder is not possible. The FTP server replies with a "550 Access denied" message if such operations are performed.
Microsoft ISA 2004 server (acts as Firewall) default settings only allow read access (list and download).
To change this settings open ISA management -> Select Firewall policy -> Web Access -> Protocol -> Filtering ->Configure FTP
On this page remove the check mark from the "Read Only" option.