I also had the same issue with online check-in for my HKG-BKK flight tonight. I had tried since T-48hr to check in, only to be given the same error message as you got. This was a ticket issued by CX TPE, on 160- stock.
I called MPC, who then referred the matter to their IT Dept. They responded yesterday that it was a technical issue, which had been resolved for my ticket. As of yesterday night, I could online checkin, and everything back to normal for me.