I can’t help but I have a similar problem to investigate.
I am developing in delphi under windows a small program to test RoonAPI.
This program works fine on my development PC but on another PC I have this error during the discovery phase (sending a UDP message):
TUDPBlockSocket.LastError=[10060],TUDPBlockSocket.LastErrorDesc=[Connection timed out]
Sorry, for this gibberish 
I will have to investigate this problem but I think the firewall configuration must be important.
dp
Edit:
Maybe this can work:
try to add in the file /usr/local/Roon/etc/roon_api.ini
[DEFAULT]
RoonCoreIP = <server>
RoonCorePort = <port>
If the discovery is not done every time, maybe this allows to bypass it.