I have received the same error on multiple occasions over the past couple of years, but in regular Economy, on TATL involving an additional carrier on the Europe side.
A year later, the same problem still persisted. When I would call the agents said it was because the partner fare code was no longer available because the partner didn't update availability in real time. Not sure I buy that, because days later the same issue persisted... And obviously years later the same issue persists...!