searched forums and came upon this thread.
Was this problem resolved as I am having the same experience. Previously had an imac g3 500mhz and upgraded to an imac g5 1.9 ghz. I backed up my libraries and tried to import messages with no success. Foregoing that, I am trying to set up mail boxes rather than smart mboxes as they are annoying but I digress. I come up with the unable to create path message and my library mail folder looks like this: