YUN REV2 fails with old NTP server

I'm playing with some new YUN REV2 connected together into a private LAN behind a firewall. Internet access is prohibited and the proposed NTP server candidates and others are inaccessible. I need to use the official local NTP server.

Unfortunaly, the official local NTP server delivers an incompatible NTP protocol, YUN REV2 returns an error "Alarm clock" and time is unset.

To implement quickly a compatible YUN REV2 NTP server, I have connected another YUN REV2 acting as NTP server (with appropriate Internet access behind the firewall to synchronize itself).