NTP 4.2.8 loses sync, system time far out

[MatthiasKoch]

Good $TIMEOFDAY,

I am running a number of virtual OpenBSD-Hosts on a VMware-vSphere, with
the stock ntpd from packages. Their ntpd.confs are pretty straightforward:

# $OpenBSD: ntpd.conf,v 1.14 2015/07/15 20:28:37 ajacoutot Exp $
#
# See ntpd.conf(5) and /etc/examples/ntpd.conf

servers pool.ntp.org
constraints from "https://www.google.com"

While all machines are syncing properly, one of them (openBSD 6.0) keeps
losing sync and drifts waaay out. A newstart of ntpd brings the system
time back on track, but after a few minutes it loses sync again and
drifts out.

ntpd ist started with ntpd_flags='-s -v'.

Here's a typical output:

Nov 15 13:57:24 n2 ntpd[9622]: ntp engine ready
Nov 15 13:57:51 n2 ntpd[94990]: set local clock to Tue Nov 15 13:57:51
CET 2016 (offset 26.320751s)
Nov 15 13:57:52 n2 ntpd[9622]: constraint reply from 172.217.16.36:
offset -0.877704
Nov 15 13:58:09 n2 ntpd[9622]: peer 52.59.88.68 now valid
Nov 15 13:58:13 n2 ntpd[9622]: peer 176.9.253.75 now valid
Nov 15 13:58:15 n2 ntpd[9622]: peer 78.46.189.152 now valid
Nov 15 13:58:18 n2 ntpd[9622]: peer 87.106.126.46 now valid
Nov 15 14:02:22 n2 ntpd[39746]: adjusting local clock by 3.956680s
Nov 15 14:02:22 n2 ntpd[9622]: clock is now synced
Nov 15 14:02:34 n2 ntpd[9622]: peer 52.59.88.68 now invalid
Nov 15 14:06:11 n2 ntpd[39746]: adjusting local clock by 3.816125s
Nov 15 14:06:11 n2 ntpd[9622]: clock is now unsynced

This is where the first "unsynced" message appears, less than 10 minutes
after restarting ntpd. From that point on, the clock stays out of sync
and veers off.

An almost identical virtual machine, running on the same host, is
unaffected.

I'm particularly wondering what's happening here:

Nov 15 14:06:11 n2 ntpd[9622]: clock is now unsynced

Any input would greatly be appreciated.

Matthias