To solve the bug is necessary to change everywhere in file file_api.php following code line:

file_ftp_delete ( $ftp, $row['filename'] );
with:
file_ftp_delete ( $ftp, $row['diskfile'] );

and:

file_ftp_delete ( $ftp, $t_filename );
with:
file_ftp_delete ( $ftp, $t_diskfile );

Infact file_add (file_api.php) write the file in FTP server using UID filename and so we have to use same name to remove it.

Fixed in CVS as part of the changes for 0004659