Can't add CalDAV calendar from iPhone

Hi!

I've been trying to solve this one for a while.

I have a MacMini running Server 10.9.5. I run my calendar from there. A little more than a month ago i stopped getting calendar updates on my iPhone 6 (9.0.1).

I deleted the calendar on the iPhone and tried to add it again, which failed, encrypted and unencrypted. I can only add it back when i'm in my local network and use the local IP. Every time i try to add it over my public, static IP it fails.

I tried this with several iPhones (9.0.1 and 9.1).

The weird thing is, when i add the calendar from a MacBook, from any network over the public IP, it works, no problem.

I checked my firewall's logs and the connections are allowed every time, even from the iPhone.

When i check the Server's console while trying to add the calendar don't see any logs about it.

I really don't get where the requests are blocked..