Hey Riccardo

Thanks for reaching out and letting us know about this. Could you give us a little more information on the email you received these links from so we can track down where the issue is? In the mean time you should be able to receive the downloads here on nagios.org from the download button, and here is a link to the installation guide.

Also just a friendly reminder to our form users to be safe when clicking links. If you are looking for downloads of our products its best to get them directly from us to ensure non-malicious links.

https://assets.nagios.com/downloads/csp/docs/Nagios-CSP-Installation-Guide.pdf

Update:
Assuming you tested the welcome to Nagios CSP email, I just tested the links and they worked. Would you mind testing the links again and see if you can access them?

Thanks,
Sandor G