Player A has desecration demon, at start of his combat opponent sacs a creature but no +1/+1 counter is placed.
On the next Player A turn, player N call the judge to inform that A forgot to place a counter.

This is not a missed trigger but a GRV, imo. Warning for player A for GRV, warning for player N for FTMGS. Agreed? The main question is, how to fix (if any).

You are correct - the trigger was not missed at all, just resolved incorrectly. I'd agree with GPE-GRV for Player A, and FtMGS for Player B.

Per the MIPG, the remedy is either back up - per the Head Judge's decision - or leave things as they are.

Thanks! – Scott Marshall, Judge Program NetRep, L5, Denver
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That's exactly how I solved it and decided that it was too complex to back up. So, no fix at all.
It would involve backing up 2 draws and casting of a creture on each side.