I don't have an N900 I assume just like any linux, and even the N800, just cd to .ssh, that is /home/user/.ssh open the file known_hosts and clear the contents. Do this on both ubuntu and the N900, and when you try to ssh next time it should ask if you want to trust the device and then automatically setup a key again. Again I don't own an N900 but this works for me on my N800 and ubuntu desktop ***EDIT*** sorry idk how much linux experience you have: you may have to be root when opening the file to be able to save it. on N900 idk what editor is available. On N800 I just use leafpad, so from the command line I would type: sudo leafpad .ssh/known_hosts and this would open the file to edit in leafpad