Most likely caused by a pfBlockerNG DNSBL entry to block AWS IPs.

Navigate to Firewall → pfBlockerNG → Reports.

To determine what it being blocked:

• Keep refreshing Amazon, and then refresh these reports.

Whitelist the following:

device-metrics-us.amazon.com
aax-us-iad.amazon.com
aan.amazon.com

mads.amazon.com #amazon mobile shopping
aax-us-iad.amazon.com #amazon mobile shopping
aan.amazon.com #amazon mobile shopping

May need to add these, but try each one individually. These are Google ad domains so probably not required.

ad.doubleclick.net
googleadservices.com
ssl.google-analytics.com
s0.2mdn.net