I am having exactly the same setup and exactly the same problem - nevertheless I would like to add some more information in order to hopefully find / get help from other readers in here.
I am using DynDNS - not a dedicated domain. I do not utilize an AirPort Extreme for port forwarding and else. All the necessary port forwardings were triple checked!
- First of all "Apple Remote Desktop" is able to connect to my server over the internet (screen sharing) and locally in my LAN. No troubles here.
- My iOS devices can connect using various VNC clients without any troubles.
- VNC connections started within Safari (MacBook Pro) do work very well, too.
- All kinds of WebDAV connections are working like a charme.
- When connecting with "Server.app" from my MacBook Pro it asks if I would like to trust the self-signed certificate before I can enter name and password. After trusting that certificate it doesn't matter which account information I am entering: none is accepted. It does not make any difference if I am using a local user account or a user network account (both configured as being allowed to administer that server). "Server.app" does not accept any information provided.
I've been following the KB advice, too that c xavier mentioned. "servermgrd" is loaded!
For some odd yet unknown reason neither WAN nor LAN FTP login does work, too. It just gives me a time out, as already mentioned WebDAV is working fine.
Sorry I could not help You, c xavier! I just thought that some additional information could help...
The exact same problem here too! Can use the Server app on the Mac mini itself via screen sharing, but cannot connect from the Server app installed on my iMac (the same computer that I'm using for the screen sharing). Both are running OS 18.8.4.
It's not the end of the world – I can just continue to use screen sharing – but it would be nice to get it sorted.
The Mac Mini has a wired connection to the router (which intern was connected to the internet) and the Macbook Pro connected to the wireless network provided by the router.
The port information can be found here: http://support.apple.com/kb/TS1629?viewlocale=en_US&locale=en_US
I don't have the ports that were forwarded at hand and I can't remember.
Once I forwarded the ports successfully I was able to connect from the Macbook Pro both from within the local network and from the internet.
Hope this helps!