I posted about the same thing 2 months ago SFO-HKG. I never was able to complete online checkin after trying with a Mac and PC (same error message as you). I mentioned it to agent while physically checking in, and she told me that the system had been having problems for 1 1/2 months.

Then while on the airplane, the passenger next to me stated that she had no problems checking in online, and showed me her boarding pass with a very low number.

Not a huge deal, but I was hoping that the issue would be sorted out before my next flight.