Rapid flashing means bad communication, SPI Loop. Bad L24 card, bad cables between the cards, are the usual culprits. Since this elevator has been in service for some time, I would lean towards the bad L24 card.
If you do not have spare L24 cards, the ones in the COP are identical. You can borrow them, run through the riser changing cards until you find the culprit, then just replace the ones in the COP when parts arrive.
