Shot in the dark here ...
What specific version of FMS13 is it .. e.g. 13.0.9?
What version are the clients.
I had this problem with FMS13.0.9 and FM12 clients, it related to a self signed SSL cert.
Here is the FM advice on it http://help.filemaker.com/app/answers/detail/a_id/14590/kw/ssl%20certificate
Thank you for the post.
If FileMaker Pro (Advanced) is installed on the physical server and can see the hosted files, the most likely port 5003 is blocked on the Windows Firewall. Try turning all Firewalls OFF for testing, and if the files can then be seen, then configure a rule (ingoing and outgoing) for port 5003 for all computers in the configuration.
Additionally, review the following knowledge for additional troubleshooting suggestions: