It will be no suprise to a man of your experience that you have a communications problem. My first suggestion would be to try different settings (baud rate, etc), then a different cable, then a different interface, etc. To confirm that the H8000 RS232 is good, you could try MIDI over DIN or FireWire. Also, consider a Clear Setup on the H8000 to reset the comms parameters.
If the above fails, it's almost certainly a computer issue. Sorry.