Webhook 503 to Build Machine

We are receiving a 503 on all requests sent to our Build machine.

We have whitelisted the IPs mentioned in this article: https://confluence.atlassian.com/bitbucket/what-are-the-bitbucket-cloud-ip-addresses-i-should-use-to-configure-my-corporate-firewall-343343385.html

(for hooks like POST)

104.192.136.0/21
34.198.203.127
34.198.32.85

When we open the machine to allow all traffic it works. Different IPs appear to being used.