Try to ping the FQDN of agent from the machine where the ZCC in browser is opened.
The "server" in the error message indicates the Remote Management server (i.e. agent ) not ZCM server.
To determine the FQDN being used:
- Try to Remote Control the agent machine using the host name.
- After Launching the RM attempt, open the source of the browser page (right click on the page and select view source)
- Search for "cmdline" in the source.
- You should see the FQDN of the machine to which you are Remote Controlling in the same line as a "cmdline" as a command line option.