Not sure... I always type the mode as "Monitor" not "monitor". Try that, see if it helps. I also found that launching kismet after setting your card to monitor mode with iwconfig, then closing kismet and trying to inject, gets me that output. I have to unload and reload the lxp drivers to get it to work again.

Oh, you also asked about macchanger: I just run "macchanger -r wlan0" after/before changing the mode, while the interface is down, both before and after injecting.