> Failed to
connect to session bus for device reservation Unable to
> autolaunch a dbus-daemon without a $DISPLAY$
The following seems to happen: when you start jackd manually, you
already have a graphical environment providing $DISPLAY which is
again used by dbus. When you start jackd during boot, it will fail
if there is no graphical environment started yet.
That's the culprit :). Forget my _wrong_ assumption about something
else grabbing the device.