Comment 14 for bug 1767322

I had the same problem and just found out that it was caused by a USB network interface that probably has been added when I plugged in a USB network device some time ago. The device has not been connected for months, yet apparently it's active in network manager, trying to get DHCP leases, which of course will fail.

Removing the USB connection profile from network manager solved the problem for me.

Sylog output example before removing connection profile:

Aug 31 07:59:43 p4t-nb-199 dhclient[31345]: DHCPDISCOVER on enp0s20f0u3c2 to 255.255.255.255 port 67 interval 5 (xid=0x5dc07171)
Aug 31 07:59:48 p4t-nb-199 dhclient[31345]: DHCPDISCOVER on enp0s20f0u3c2 to 255.255.255.255 port 67 interval 13 (xid=0x5dc07171)
Aug 31 08:00:01 p4t-nb-199 dhclient[31345]: DHCPDISCOVER on enp0s20f0u3c2 to 255.255.255.255 port 67 interval 16 (xid=0x5dc07171)
Aug 31 08:00:17 p4t-nb-199 dhclient[31345]: DHCPDISCOVER on enp0s20f0u3c2 to 255.255.255.255 port 67 interval 13 (xid=0x5dc07171)
Aug 31 08:00:24 p4t-nb-199 NetworkManager[1026]: <warn> [1535695224.9894] dhcp4 (enp0s20f0u3c2): request timed out
Aug 31 08:00:24 p4t-nb-199 NetworkManager[1026]: <info> [1535695224.9895] dhcp4 (enp0s20f0u3c2): state changed unknown -> timeout
Aug 31 08:00:25 p4t-nb-199 NetworkManager[1026]: <info> [1535695225.0064] dhcp4 (enp0s20f0u3c2): canceled DHCP transaction, DHCP client pid 31345
Aug 31 08:00:25 p4t-nb-199 NetworkManager[1026]: <info> [1535695225.0066] dhcp4 (enp0s20f0u3c2): state changed timeout -> done
Aug 31 08:00:25 p4t-nb-199 NetworkManager[1026]: <info> [1535695225.0083] device (enp0s20f0u3c2): state change: ip-config -> failed (reason 'ip-config-unavailable', sys-iface-state: 'managed')
Aug 31 08:00:25 p4t-nb-199 NetworkManager[1026]: <warn> [1535695225.0122] device (enp0s20f0u3c2): Activation: failed for connection 'Wired connection 1'
Aug 31 08:00:25 p4t-nb-199 NetworkManager[1026]: <info> [1535695225.0160] device (enp0s20f0u3c2): state change: failed -> disconnected (reason 'none', sys-iface-state: 'managed')
Aug 31 08:00:25 p4t-nb-199 avahi-daemon[1009]: Withdrawing address record for fe80::dc1a:366d:4591:9ef8 on enp0s20f0u3c2.
Aug 31 08:00:25 p4t-nb-199 avahi-daemon[1009]: Leaving mDNS multicast group on interface enp0s20f0u3c2.IPv6 with address fe80::dc1a:366d:4591:9ef8.
Aug 31 08:00:25 p4t-nb-199 systemd-timesyncd[793]: Network configuration changed, trying to establish connection.
Aug 31 08:00:25 p4t-nb-199 avahi-daemon[1009]: Interface enp0s20f0u3c2.IPv6 no longer relevant for mDNS.
Aug 31 08:00:25 p4t-nb-199 NetworkManager[1026]: <info> [1535695225.0415] policy: auto-activating connection 'Wired connection 1'
Aug 31 08:00:25 p4t-nb-199 NetworkManager[1026]: <info> [1535695225.0441] device (enp0s20f0u3c2): Activation: starting connection 'Wired connection 1' (469a98cb-89f1-384c-bae1-f72617a41bff)
Aug 31 08:00:25 p4t-nb-199 NetworkManager[1026]: <info> [1535695225.0446] device (enp0s20f0u3c2): state change: disconnected -> prepare (reason 'none', sys-iface-state: 'managed')
Aug 31 08:00:25 p4t-nb-199 NetworkManager[1026]: <info> [1535695225.0457] device (enp0s20f0u3c2): state change: prepare -> config (reason 'none', sys-iface-state: 'managed')
Aug 31 08:00:25 p4t-nb-199 NetworkManager[1026]: <info> [1535695225.0470] device (enp0s20f0u3c2): state change: config -> ip-config (reason 'none', sys-iface-state: 'managed')
Aug 31 08:00:25 p4t-nb-199 NetworkManager[1026]: <info> [1535695225.0475] dhcp4 (enp0s20f0u3c2): activation: beginning transaction (timeout in 45 seconds)
Aug 31 08:00:25 p4t-nb-199 systemd-timesyncd[793]: Synchronized to time server 91.189.94.4:123 (ntp.ubuntu.com).
Aug 31 08:00:25 p4t-nb-199 NetworkManager[1026]: <info> [1535695225.0532] dhcp4 (enp0s20f0u3c2): dhclient started with pid 31373
Aug 31 08:00:25 p4t-nb-199 dhclient[31373]: DHCPDISCOVER on enp0s20f0u3c2 to 255.255.255.255 port 67 interval 3 (xid=0x3463f46)
Aug 31 08:00:26 p4t-nb-199 avahi-daemon[1009]: Joining mDNS multicast group on interface enp0s20f0u3c2.IPv6 with address fe80::dc1a:366d:4591:9ef8.
Aug 31 08:00:26 p4t-nb-199 avahi-daemon[1009]: New relevant interface enp0s20f0u3c2.IPv6 for mDNS.
Aug 31 08:00:26 p4t-nb-199 systemd-networkd[794]: enp0s20f0u3c2: Gained IPv6LL
Aug 31 08:00:26 p4t-nb-199 avahi-daemon[1009]: Registering new address record for fe80::dc1a:366d:4591:9ef8 on enp0s20f0u3c2.*.
Aug 31 08:00:26 p4t-nb-199 systemd-timesyncd[793]: Network configuration changed, trying to establish connection.
Aug 31 08:00:26 p4t-nb-199 systemd-timesyncd[793]: Synchronized to time server 91.189.94.4:123 (ntp.ubuntu.com).