Dovecot says time moves backwards

Yesterday suddenly this started appearing in mail.log:

Code:

Jun 15 18:07:36 posti dovecot: log: Warning: Time moved backwards by 3495 seconds.
Jun 15 18:07:36 posti dovecot: imap: Fatal: Time just moved backwards by 3484 seconds. This might cause a lot of problems, so I'll just kill myself now. http://wiki2.dovecot.org/TimeMovedBackwards
Jun 15 18:07:36 posti dovecot: master: Warning: Time moved backwards by 3496 seconds, waiting for 180 secs until new services are launched again.
Jun 15 18:07:36 posti dovecot: imap-login: Warning: Time moved backwards by 3450 seconds.
Jun 15 18:07:36 posti dovecot: anvil: Warning: Time moved backwards by 3496 seconds.
Jun 15 18:07:36 posti dovecot: config: Warning: Time moved backwards by 3496 seconds.
Jun 15 18:07:36 posti dovecot: pop3-login: Warning: Time moved backwards by 3513 seconds.
Jun 15 18:07:36 posti dovecot: pop3: Fatal: Time just moved backwards by 3513 seconds. This might cause a lot of problems, so I'll just kill myself now. http://wiki2.dovecot.org/TimeMovedBackwards
Jun 15 18:07:36 posti dovecot: auth: Warning: Time moved backwards by 3496 seconds.
Jun 15 18:07:36 posti dovecot: ssl-params: Warning: Time moved backwards by 3495 seconds.
Jun 15 18:07:36 posti dovecot: imap: Fatal: Time just moved backwards by 3484 seconds. This might cause a lot of problems, so I'll just kill myself now. http://wiki2.dovecot.org/TimeMovedBackwards
Jun 15 18:07:36 posti dovecot: imap: Fatal: Time just moved backwards by 3484 seconds. This might cause a lot of problems, so I'll just kill myself now. http://wiki2.dovecot.org/TimeMovedBackwards
Jun 15 18:07:36 posti dovecot: imap-login: Warning: Time moved backwards by 3484 seconds.
Jun 15 18:07:36 posti dovecot: imap-login: Warning: Time moved backwards by 3484 seconds.

It stopped after a while:

Code:

Jun 15 18:23:46 posti dovecot: imap-login: Warning: Time moved backwards by 1714 seconds.
Jun 15 18:23:46 posti dovecot: imap: Fatal: Time just moved backwards by 1719 seconds. This might cause a lot of problems, so I'll just kill myself now. http://wiki2.dovecot.org/TimeMovedBackwards
Jun 15 18:23:46 posti dovecot: imap-login: Warning: Time moved backwards by 1714 seconds.
Jun 15 18:23:47 posti dovecot: imap: Fatal: Time just moved backwards by 1718 seconds. This might cause a lot of problems, so I'll just kill myself now. http://wiki2.dovecot.org/TimeMovedBackwards
Jun 15 18:23:47 posti dovecot: imap-login: Warning: Time moved backwards by 1714 seconds.
Jun 15 18:36:29 posti dovecot: imap: Fatal: Time just moved backwards by 1714 seconds. This might cause a lot of problems, so I'll just kill myself now. http://wiki2.dovecot.org/TimeMovedBackwards
Jun 15 18:36:29 posti dovecot: imap: Fatal: Time just moved backwards by 1714 seconds. This might cause a lot of problems, so I'll just kill myself now. http://wiki2.dovecot.org/TimeMovedBackwards

This host is virtual host running on Proxmox 5. Not a container, full virtual host. Not running ntpdate on ntp on the virtual host.
I read the dovevot wiki page, but do not understand why this would happen on this host. Any idea what caused it and what to do?

 

I found time jump forwards also:

Code:

# grep forwards /var/log/mail.log
Jun 15 17:58:24 posti dovecot: imap: Warning: Time jumped forwards 3513 seconds
Jun 15 17:58:25 posti dovecot: imap: Warning: Time jumped forwards 3513 seconds
Jun 15 17:58:25 posti dovecot: imap: Warning: Time jumped forwards 3513 seconds
Jun 15 17:58:25 posti dovecot: imap: Warning: Time jumped forwards 3487 seconds
Jun 15 17:58:25 posti dovecot: imap: Warning: Time jumped forwards 3488 seconds
Jun 15 17:58:29 posti dovecot: imap: Warning: Time jumped forwards 3514 seconds
Jun 15 17:58:33 posti dovecot: imap: Warning: Time jumped forwards 3514 seconds
Jun 15 17:58:39 posti dovecot: imap: Warning: Time jumped forwards 3514 seconds
Jun 15 17:58:40 posti dovecot: imap: Warning: Time jumped forwards 3514 seconds
Jun 15 17:58:41 posti dovecot: imap: Warning: Time jumped forwards 3514 seconds
Jun 15 17:58:41 posti dovecot: imap: Warning: Time jumped forwards 3513 seconds
Jun 15 17:58:41 posti dovecot: imap: Warning: Time jumped forwards 3513 seconds
Jun 15 17:58:42 posti dovecot: imap: Warning: Time jumped forwards 3513 seconds
Jun 15 17:58:42 posti dovecot: imap: Warning: Time jumped forwards 3513 seconds
Jun 15 17:58:42 posti dovecot: imap: Warning: Time jumped forwards 3514 seconds
Jun 15 17:58:44 posti dovecot: imap: Warning: Time jumped forwards 3514 seconds
Jun 15 17:58:44 posti dovecot: imap: Warning: Time jumped forwards 3513 seconds
Jun 15 17:58:44 posti dovecot: imap: Warning: Time jumped forwards 3513 seconds
Jun 15 17:58:44 posti dovecot: imap: Warning: Time jumped forwards 3513 seconds
Jun 15 17:58:45 posti dovecot: imap: Warning: Time jumped forwards 3513 seconds
Jun 15 17:58:46 posti dovecot: imap: Warning: Time jumped forwards 3513 seconds
Jun 15 17:58:49 posti dovecot: imap: Warning: Time jumped forwards 3514 seconds
Jun 15 17:58:50 posti dovecot: imap: Warning: Time jumped forwards 3513 seconds
Jun 15 17:58:50 posti dovecot: imap: Warning: Time jumped forwards 3513 seconds
Jun 15 17:59:39 posti dovecot: imap: Warning: Time jumped forwards 1829 seconds
Jun 15 17:59:42 posti dovecot: imap: Warning: Time jumped forwards 1984 seconds
Jun 15 18:05:19 posti dovecot: imap: Warning: Time jumped forwards 2593 seconds
Jun 15 18:05:19 posti dovecot: imap: Warning: Time jumped forwards 2592 seconds
Jun 15 18:18:23 posti dovecot: imap: Warning: Time jumped forwards 3513 seconds
Jun 15 18:20:03 posti dovecot: imap: Warning: Time jumped forwards 3513 seconds
Jun 15 18:20:03 posti dovecot: imap: Warning: Time jumped forwards 3513 seconds

The timestamps in mail.log do not jump back and forth, so the time the log application sees flows steadily forwards. I do not understand why dovecot thinks time is jumping willy nilly here and there.

 

They are not containeres, kvm virtual hosts. This has not happened before, and I have tried to configure the timekeeping on Proxmox properly.
But, now that I look at systemctl status systemd-timesyncd it shows

Code:

Jun 14 18:09:16 ns61053275 systemd-timesyncd[16190]: Network configuration changed, trying to establish connection.
Jun 14 18:09:16 ns61053275 systemd-timesyncd[16190]: Synchronized to time server 213.251.128.249:123 (ntp.ovh.net).
Jun 14 18:15:16 ns61053275 systemd-timesyncd[16190]: Network configuration changed, trying to establish connection.
Jun 14 18:15:16 ns61053275 systemd-timesyncd[16190]: Synchronized to time server 213.251.128.249:123 (ntp.ovh.net).
Jun 14 18:15:17 ns61053275 systemd-timesyncd[16190]: Network configuration changed, trying to establish connection.
Jun 14 18:15:17 ns61053275 systemd-timesyncd[16190]: Synchronized to time server 213.251.128.249:123 (ntp.ovh.net).
Jun 14 18:17:56 ns61053275 systemd-timesyncd[16190]: Network configuration changed, trying to establish connection.
Jun 14 18:17:56 ns61053275 systemd-timesyncd[16190]: Synchronized to time server 213.251.128.249:123 (ntp.ovh.net).
Jun 14 18:17:57 ns61053275 systemd-timesyncd[16190]: Network configuration changed, trying to establish connection.
Jun 14 18:17:57 ns61053275 systemd-timesyncd[16190]: Synchronized to time server 213.251.128.249:123 (ntp.ovh.net).

Looks like something happened yesterday in providers network that caused this issue with timesync.