Do you have a split reservation, changing rooms or resort at some point or reservation was made with two different use years and then linked? The system apparently does not allow on-line check-in in those cases.
We are arriving on the 22nd as well and I was able to do online check-in this morning. Must have gone down later on in the day. Good luck, I would just keep trying. And, have a great time. Hope we have good weather!