Originally Posted by
MGStickler
It was not really an error message. Is said something to the effect "Unable to complete the check-in process. Please proceed to the airport for check in".
I got this exact same reply for hours when I tried to check in for my international flight to SYD last month. It suddenly dawned on me that I had never filed for my annual ETA (electronic VISA). I went online, filed and paid for it.
5 minutes later I went back to UA's web site and voila, I was able to do online check in. ^