Seems a systemwide thing...
I also couldn't check in either via mobile or website. Called the 1K line and only had to wait a minute or two before I got an agent. The poor agent couldn't find the record a couple of different ways at first, but then found it. (I think I waited longer for her to find the record than I waited listening to the annoying hold ads.)
Nothing wrong with the record. Agent tried to acomplish check-in on her end and it came up with the same error message that I got (and all the others on this thread). So, we agreed that I would try again later today.
Here's hoping...