Syncthing daemon starts manually using script, but not at startup

I have installed syncthing on a Linux Mint Debian Edition machine and am trying to get it started at boot time using an init script in /etc/init.d. I followed this tutorial. Symlinks in /etc/rc.* exist and I can start and stop the daemon just fine when I execute the script manually (as root). However, the script is not started at boot time.

This is the script:

#!/bin/sh
### BEGIN INIT INFO
# Provides: syncthing
# Required-Start: $local_fs $remote_fs
# Required-Stop: $local_fs $remote_fs
# Should-Start: $network
# Should-Stop: $network
# Default-Start: 2 3 4 5
# Default-Stop: 0 1 6
# Short-Description: Multi-user daemonized version of syncthing.
# Description: Starts the syncthing daemon for all registered users.
### END INIT INFO

# Replace with users you want to run syncthing clients for
syncthing_USERS="XXXXXX"
DAEMON=/opt/syncthing-linux-amd64-v0.10.8/syncthing

echo "This is /etc/init.d/syncthing" > /tmp/syncthing.txt

startd() {
  echo "Trying to start daemons..." >> /tmp/syncthing.txt
  for stuser in $syncthing_USERS; do
    echo "Trying $stuser"  >> /tmp/syncthing.txt
    HOMEDIR=$(getent passwd $stuser | awk -F: '{print $6}')
    echo "HOMEDIR = $HOMEDIR"  >> /tmp/syncthing.txt
    echo "config = $config"  >> /tmp/syncthing.txt
    if [ -f $config ]; then
      echo "Starting syncthiing for $stuser"
      start-stop-daemon -b -o -c $stuser -S -u $stuser -x $DAEMON
    else
      echo "Couldn't start syncthing for $stuser (no $config found)"
    fi
  done
}

stopd() {
  for stuser in $syncthing_USERS; do
    dbpid=$(pgrep -fu $stuser $DAEMON)
    if [ ! -z "$dbpid" ]; then
      echo "Stopping syncthing for $stuser"
      start-stop-daemon -o -c $stuser -K -u $stuser -x $DAEMON
    fi
  done
}

status() {
  for stuser in $syncthing_USERS; do
    dbpid=$(pgrep -fu $stuser $DAEMON)
    if [ -z "$dbpid" ]; then
      echo "syncthing for USER $stuser: not running."
    else
      echo "syncthing for USER $stuser: running (pid $dbpid)"
    fi
  done
}

case "$1" in
  start) startd
    ;;
  stop) stopd
    ;;
  restart|reload|force-reload) stopd && startd
    ;;
  status) status
    ;;
  *) echo "Usage: /etc/init.d/syncthing {start|stop|reload|force-reload|restart|status}"
     exit 1
   ;;
esac

exit 0

Here are the symlinks:

$find /etc/rc* -name "*syncthing*"
/etc/rc0.d/K01syncthing
/etc/rc1.d/K01syncthing
/etc/rc2.d/S01syncthing
/etc/rc3.d/S01syncthing
/etc/rc4.d/S01syncthing
/etc/rc5.d/S01syncthing
/etc/rc6.d/K01syncthing

and my current runlevel is

$/sbin/runlevel
N 2

Note that I have inserted several echo statements at the very top, which are piped into a file in /tmp for debugging. Surprisingly, the script is executed at boot time but the daemon doesn't start, and the file reads

This is /etc/init.d/syncthing
Trying to start daemons...
Trying XXXXXX
HOMEDIR = /home/XXXXXX
config = 

When started manually by executing the script as root after booting, the output is also created and the output is identical. What's happening?

Something else I don't understand is that the script tests for a file $config, but this variable is never defined and so the file never exists. But sometimes this test seems to evaluate to true and sometimes false?

What am I missing?