udev rule not working

I've set up backintime to work when a drive gets connected. However, it doesn't appear to be working properly. I originally tried with root, since I wish to back up my entire file system. I also tried a minimal situation for my normal user, but this also fails. I'm using KDE Plasma 5.3.1 in Arch Linux.

When I plug in my hard drive, a udev rule appears to activate, because a notification pops up. This says that it cannot find the hard drive, and will wait 30 seconds. Presumably this is because the hard drive is not mounted until I click mount in the Plasma's tray popup. I mount the hard drive within a few seconds, but the backup never starts.

~/.config/backintime/config is here: http://pastebin.com/evrkYs80

/etc/udev/rules.d/99-backintime-sparhawk.rules comprises the following line.

ACTION=="add", ENV{ID_FS_UUID}=="1bf6eef9-f221-4401-a00a-e37b455c8fab", RUN+="/bin/su - 'sparhawk' -c '/usr/bin/nice -n 19 /usr/bin/ionice -c2 -n7 /usr/bin/backintime --backup-job >/dev/null 2>&1'"

There is also /etc/udev/rules.d/99-backintime-root.rules which is almost identical except for the the username changing.

I cannot find any kind of related log file in /var/log