I've got it!
The sequence of events that leads to ports 26 and 29 failing the powr-up test is this:
1. Reload is requested (from CLI or menu, telnet or console) => all front panel lights are on
2. You have electrical power problems during this specific time (when all lights re on) - I unplugged the switch for my tests
3. power comes back
4. ports 26 and 29 are faulty, blinking in phase with the Fault LED.
I repeated this 5 times and the outcome was the same.
Yesterday we did have short electrical power fluctuations - so this explains the strange bahaviour of the switch.