ntpd logging & debugging

I have a FreeBSD 10.2-RELEASE server running as a KVM guest and doesn't keep its clock synchronize. I have enabled ntpd in /etc/rc.conf:
Code:
    ntpd_enable=YES
    ntpd_sync_on_start=YES
but it doesn't solve my problem. If the server runs for a few hours the difference between real time and server time is already in a range of minutes.

I have the feeling that ntpd doesn't work as expected. How can I "debug" ntpd? How can I find out when a successful / failed ntp update took place?
I have enabled logging in file /etc/ntp.conf with:
Code:
logfile /var/log/ntp.log
but the log file only tells me about the binding address which does not help me.
 
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