Are you currently running Advanced Security features on the MX? (Content Filtering, Threat Protection, AMP, IDS/IPS?) Also what version of firmware are you running on the MX?
If possible try disabling them one by one to see which is causing the traffic to be blocked.
You'll have to take a packet capture from the LAN interface of the MX and find out what domains the FTP download is coming from and then add it as a whitelist rule.
Eliot F | Simplifying IT with Cloud Solutions
Found this helpful? Give me some Kudos! (click on the little up-arrow below)