The setup

My debian server at online.net is running a dual stack ipv4/ipv6

# /etc/network/interfaces

auto lo
iface lo inet loopback

source /etc/network/interfaces.d/*

There are 2 files in /etc/network/interfaces.d/

# /etc/network/interfaces.d/device-enp0s20-inet

auto enp0s20
iface enp0s20 inet dhcp

and

# /etc/network/interfaces.d/device-enp0s20-inet6

auto enp0s20
iface enp0s20 inet6 dhcp
  request_prefix 1
  accept_ra 2

I request my ipv6 delegated prefix using dhclient

# /etc/dhcp/dhclient.conf

option rfc3442-classless-static-routes code 121 = array of unsigned integer 8;
send host-name = gethostname();
request subnet-mask, broadcast-address, time-offset, routers,
    domain-name, domain-name-servers, domain-search, host-name,
    dhcp6.name-servers, dhcp6.domain-search, dhcp6.fqdn, dhcp6.sntp-servers,
    netbios-name-servers, netbios-scope, interface-mtu,
    rfc3442-classless-static-routes, ntp-servers;


interface "enp0s20" {                                                                                                                                                                                 
  send dhcp6.client-id XX:XX:XX:XX:XX:XX:XX:XX:XX:XX;
}

In the inet6 setup of enp0s20 I want accept_ra=2, because I have net.ipv6.conf.all.forwarding=1 in my kernel (I intend to setup a vpn).

The problem

Restarting the networking service fails every second attempt.

$ sudo serving networking restart # first attempt
Job for networking.service failed because a timeout was exceeded.
See "systemctl status networking.service" and "journalctl -xe" for details.
$ sudo service networking restart # second attempt works fine (this cycles)

Starting the networking service also always fails at boot.

The logs

I enabled this to better see what is happening

# /etc/default/networking
VERBOSE=yes

Here is what I get from journalctl when restarting fails:

Apr 03 12:38:37 boxname ifup[4135]: ifup: parsing file /etc/network/interfaces.d/device-enp0s20-inet
Apr 03 12:38:37 boxname ifup[4135]: ifup: parsing file /etc/network/interfaces.d/device-enp0s20-inet6
Apr 03 12:38:37 boxname ifup[4135]: /bin/run-parts --exit-on-error --verbose /etc/network/if-pre-up.d
Apr 03 12:38:37 boxname ifup[4135]: ifup: configuring interface enp0s20=enp0s20 (inet)
Apr 03 12:38:37 boxname ifup[4135]: /bin/run-parts --exit-on-error --verbose /etc/network/if-pre-up.d
Apr 03 12:38:37 boxname ifup[4135]: /sbin/dhclient -4 -v -pf /run/dhclient.enp0s20.pid -lf /var/lib/dhcp/dhclient.enp0s20.leases -I -df /var/lib/dhcp/dhclient6.enp0s20.leases enp0s20
Apr 03 12:38:37 boxname ifup[4135]: Internet Systems Consortium DHCP Client 4.3.5
Apr 03 12:38:37 boxname ifup[4135]: Copyright 2004-2016 Internet Systems Consortium.
Apr 03 12:38:37 boxname ifup[4135]: All rights reserved.
Apr 03 12:38:37 boxname ifup[4135]: For info, please visit https://www.isc.org/software/dhcp/
Apr 03 12:38:38 boxname ifup[4135]: Listening on LPF/enp0s20/<REDACTED>
Apr 03 12:38:38 boxname ifup[4135]: Sending on   LPF/enp0s20/<REDACTED>
Apr 03 12:38:38 boxname ifup[4135]: Sending on   Socket/fallback
Apr 03 12:38:38 boxname ifup[4135]: DHCPDISCOVER on enp0s20 to 255.255.255.255 port 67 interval 4
Apr 03 12:38:38 boxname ifup[4135]: DHCPREQUEST of 163.172.XX.XX on enp0s20 to 255.255.255.255 port 67
Apr 03 12:38:38 boxname ifup[4135]: DHCPOFFER of 163.172.XX.XX from 163.172.XX.1
Apr 03 12:38:38 boxname ifup[4135]: DHCPACK of 163.172.XX.XX from 163.172.XX.1
Apr 03 12:38:38 boxname ifup[4135]: bound to 163.172.XX.XX -- renewal in 2147483648 seconds.
Apr 03 12:38:38 boxname ifup[4135]: /bin/run-parts --exit-on-error --verbose /etc/network/if-up.d
Apr 03 12:38:38 boxname ifup[4135]: run-parts: executing /etc/network/if-up.d/000resolvconf
Apr 03 12:38:38 boxname ifup[4135]: run-parts: executing /etc/network/if-up.d/openntpd
Apr 03 12:38:38 boxname ifup[4135]: run-parts: executing /etc/network/if-up.d/openssh-server
Apr 03 12:38:38 boxname ifup[4135]: run-parts: executing /etc/network/if-up.d/upstart
Apr 03 12:38:38 boxname ifup[4135]: ifup: configuring interface enp0s20=enp0s20 (inet6)
Apr 03 12:38:38 boxname ifup[4135]: /bin/run-parts --exit-on-error --verbose /etc/network/if-pre-up.d
Apr 03 12:38:38 boxname ifup[4135]: /sbin/modprobe -q net-pf-10 > /dev/null 2>&1 || true # ignore failure.
Apr 03 12:38:38 boxname ifup[4135]: /sbin/sysctl -q -e -w net.ipv6.conf.enp0s20.accept_ra=2
Apr 03 12:38:38 boxname ifup[4135]: /bin/ip link set dev enp0s20  up
Apr 03 12:38:38 boxname ifup[4135]: /lib/ifupdown/wait-for-ll6.sh
Apr 03 12:38:39 boxname ifup[4135]: /sbin/dhclient -6 -pf /run/dhclient6.enp0s20.pid -lf /var/lib/dhcp/dhclient6.enp0s20.leases -I -P -N -df /var/lib/dhcp/dhclient.enp0s20.leases enp0s20
Apr 03 12:38:50 boxname ifup[4135]: RTNETLINK answers: Invalid argument

The question

Can you help me find out what is wrong and fix it?