[support] strange problem with direct link to a file

Hi, I have d5 and upload set to private mode. I uploaded a file and its path is: www.spazioausili.net/system/files/pcciechi_2008-14.zip the strange thing is that about 80% of users have no problem download it. but, other users, receive an error message that say that the website is temporary unavailable, or download start and crash after 1 second. This is not random, it happens always to same users. instead, if I give the link of the node and then users click to the attachment, the problem doesn't exist and all can download. do you know if drupal has some difficulties when a user wants access directly to a file when private mode is enabled? Can I solve this problem? Thanks in advance.