Zero Bytes

I am having difficulties uploading to my site. When I select the local file the upload dialogue tells me there are zero bytes in the file but the local directory listing is showing the correct size. If I proceed the old file on the site does get overwritten but shows zero bytes. I am wondering what might be causing this. There is something strange about the site which is being hosted by an ISP we have just transferred to, in that other FTP applications have difficulty listing the directory. The permissions on the folders and files are all 644 so I don't think it is a permission issue. I haven't changed any of the defailt settings in Smart FTP.

Any thoughts?