It MAY be the same problem cheve had with mounting - being too hasty. until i learned to click "VBUST ON" 2-3 seconds after plugging cable, and "Enumerate" after another 6-7 seconds, many times i get same errors.

Also, remember to always unplug device (be it HUB, stick, mouse, whatever) from N900 before trying to start from scratch. Cable and adapter can sit on N900, but nothing can be connected to them. You can connect anything only after setting speed (then, do not exceed 3-4 seconds before You click VBUS ON, but also don't do it 0.1 second after plugging device - not too late, not too early), otherwise you get exact same errors. At least, that is in my case, always, 100% reproduceable.

On side note, i would NOT recommend plugging and unplugging again cable direct from N900 when You want start "hostmod(d)ing" from scratch - it's better to unplug device/adapter from end of the cable. We don't want to stress too much our N900 USB port...