Apparently when I remove ntp and install openntpd, then the problem is solved, systemd doesn't start its own timesync demon.
- Bug#889290: ntp and systemd-timesyncd run in parallel Fejes József
- Bug#889290: Fejes József
Apparently when I remove ntp and install openntpd, then the problem is solved, systemd doesn't start its own timesync demon.