puma lock out

This is a puma 80 with permanent pilot. The heating can be on for hours without problem. Hot water can be drawn at full capacity without a problem. A little while AFTER drawing hot water when the tap has been shut is when the boiler locks out. Short bursts of hot water have no adverse effect, anywhere from one second to a couple of minutes. The problem appears only after a long shower. The system has been powerflushed and there is no sign of sludge or scale. The diverter valve has been checked and is clean and appears to work fine. The flowswitch and the micro switch have been checked and seem to work fine. Flow and return differential is about 12 degrees and all radiators heat up equally. There is no pressure loss (stays between 1 and 1.5 bar) in the heating system and no leaking taps. The fan, pcb, aav, prv and the overheat stat have been replaced and the system has sufficient inhibitor. The pilot assembly and thermocouple have been checked and work fine. The boiler has been serviced every year. The heating works fine with stat on high as well as low and on full capacity as well as rated down to minimum. Are there any secret parts that can fail in this boiler that are not listed in the manual? Lol.

if the fins through the heat exchanger are restricted with debris you can get the very problem you're experiencing. Run the hot water for a while and look at the pilot light. Has it started to look different (yellow and weak)?

Thanks for your suggestion. The boiler has been serviced 2 weeks ago and is clean as a whistle. The pilot light is blue and clearly defined. One side heats the thermocouple and one side is 2 mm above the main burner. You can even see the difference between the core and the darker mantle. The thermocouple works fine, about 20 seconds between extinguishing and the click. All radiators are bled and there is no problem whatsoever with the heating. What baffles me is that the lockout only appears AFTER a shower. Not during the shower or after drawing just a bucket full.