systemd-resolved and systemd-timesyncd timeout failure, keep trying to restart

After a recent update my 18.04 server is running in a degraded state (as reported by systemctl status) because of two failed units. In /var/log/syslog I get these lines every 90 s:

Jun 10 22:07:49 olympus systemd[1]: systemd-resolved.service: Start operation timed out. Terminating.
Jun 10 22:07:49 olympus systemd[1]: systemd-timesyncd.service: Start operation timed out. Terminating.

In journalctl -xe:

-- Unit systemd-timesyncd.service has failed.
-- 
-- The result is RESULT.
jun 10 22:22:52 olympus systemd[1]: systemd-resolved.service: Failed with result 'timeout'.
jun 10 22:22:52 olympus systemd[1]: Failed to start Network Name Resolution.

systemctl status systemd-timesyncd shows that it is Active (activating) all the time:

● systemd-timesyncd.service - Network Time Synchronization
   Loaded: loaded (/lib/systemd/system/systemd-timesyncd.service; enabled; vendor preset: enabled)
   Active: activating (start) since Mon 2019-06-10 22:34:54 CEST; 1min 4s ago
     Docs: man:systemd-timesyncd.service(8)
 Main PID: 14410 ((imesyncd))
    Tasks: 1 (limit: 4302)
   CGroup: /system.slice/systemd-timesyncd.service
           └─14410 (imesyncd)

jun 10 22:34:54 olympus systemd[1]: Starting Network Time Synchronization...

After 1m 30s it gives up and restarts. The same goes for systemd-resolvd.

Apart from this activity (and the huge number of messages it generates) the server appears to work well.

What is really wrong with these units? How can I debug the problem?

EDIT: output of requested commands. There is nothing in /etc/netplan/ nor in /etc/network/interfaces.d/.

jos@olympus:~$ ls -al /etc/resolv.conf
-rw-r--r-- 1 root root 65 jan 30 09:46 /etc/resolv.conf
jos@olympus:~$ cat /etc/resolv.conf
nameserver 192.168.124.254
nameserver 8.8.8.8
nameserver 8.8.4.4
jos@olympus:~$ ls /etc/netplan
jos@olympus:~$ cat /etc/network/interfaces
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

source /etc/network/interfaces.d/*

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto enp4s0
iface enp4s0 inet static
    address 192.168.124.99
    netmask 255.255.255.0
    gateway 192.168.124.254
    dns-nameservers 192.168.124.254 213.75.63.75 213.75.63.76 8.8.8.8 8.8.4.4