@gohan
New Arduino wired and conncted, uploaded sketch, etc.
Same result.

@martinhjelmare
I changed the hass cfg to:

417 mysensors:
418   gateways:
419     - device: mqtt
420       persistence_file: '/home/homeassistant/.homeassistant/mysensors.json'
421       topic_in_prefix: 'mysensors-out'
422       topic_out_prefix: 'mysensors-in'

Now its the same as reported in the gateway log...

Its working. Thanks guys!

One last question, just to be clear:
Its fine to have "#define CHILD_ID 0", right?