Why does Slurm fail to start with systemd but work when starting manually?

I've just set up slurm where one physical machine will be the only system in the cluster (so far). This is on Ubuntu 18.04.

I have slurmdbd running, but when I attempt to start up slurmd and slurmctld this times out. Why?

I'm issuing the following commands:

systemctl start slurmctld
systemctl start slurmd

I've also tried:

systemctl start slurmctld slurmd

and:

systemctl start slurmd slurmctld

This fails with the following, for slurmctld:

systemd[1]: slurmd.service: Can't open PID file /var/run/slurm-llnl/slurm-llnl/slurmd.pid (yet?) after start: No such file or directory
systemd[1]: slurmctld.service: Start operation timed out. Terminating.
systemd[1]: slurmctld.service: Failed with result 'timeout'.
systemd[1]: Failed to start Slurm controller daemon.

And for slurmd:

systemd[1]: slurmd.service: Start operation timed out. Terminating.
systemd[1]: slurmd.service: Failed with result 'timeout'.
systemd[1]: Failed to start Slurm node daemon.

However, when I start these manually (using two terminals) by issuing:

slurmctld -Dvvv
slurmd -Dvvv

Everything appears to work.

Why is this? How am I supposed to start slurm?

These are the service files (which should be standard, I didn't touch them except for adding verbose arguments, but then removing them again later):

# cat /lib/systemd/system/slurmd.service 
[Unit]
Description=Slurm node daemon
After=network.target munge.service
ConditionPathExists=/etc/slurm-llnl/slurm.conf
Documentation=man:slurmd(8)

[Service]
Type=forking
EnvironmentFile=-/etc/default/slurmd
ExecStart=/usr/sbin/slurmd $SLURMD_OPTIONS
ExecReload=/bin/kill -HUP $MAINPID
PIDFile=/var/run/slurm-llnl/slurmd.pid
KillMode=process
LimitNOFILE=51200
LimitMEMLOCK=infinity
LimitSTACK=infinity

[Install]
WantedBy=multi-user.target
# cat /lib/systemd/system/slurmctld.service 
[Unit]
Description=Slurm controller daemon
After=network.target munge.service
ConditionPathExists=/etc/slurm-llnl/slurm.conf
Documentation=man:slurmctld(8)

[Service]
Type=forking
EnvironmentFile=-/etc/default/slurmctld
ExecStart=/usr/sbin/slurmctld $SLURMCTLD_OPTIONS
ExecReload=/bin/kill -HUP $MAINPID
PIDFile=/var/run/slurm-llnl/slurmctld.pid

[Install]
WantedBy=multi-user.target