This article will explain how to fix the issue of downloading Signature Database when the proxy involved.

If downloading Signature Database is failing with the error "URL is not accessible for url https://signatures.juniper.net/space/2/latest/manifest.xml", you can follow the KB: [Security Director] IPS Signatures fail to download only if we are seeing "Certificate not trusted" error in server.log. Otherwise, this article will helps to fix the issue

To identify the issue, please run the tcpdump on Junos space with proxy IP and -w flag to export it to PCAP file by downloading the Signature Database from Security director.

You will see "Handshake Failure: error as per the screenshot below

 
As per the above screenshot, Junos Space is sending the client hello and the proxy server is unable to make a handshake during SSL handshaking.

To fix the issue, please contact proxy server Team from your end.