I've had that on an international booking the past few weeks and finally used the chat feature which , unlike my previous experiences, worked great today. I got $900 eCredit and a better route. I'm not sure why the error comes up - the only thing non-standard about my booking was that I had used the thing where you get $50 off per 5000 Skymiles - not sure where those Skymiles ended up now, and maybe they are part of the $900 credit so I'll take a look. So maybe use of Skymiles triggers the error, not sure