This is great advice. A Web Support agent should definitely be able to help explain the problem.

The most recent time I saw something like this happen personally was on the return portion of a round-trip British Airways flight; we had a two-passenger itinerary and only one passenger was able to check in online, while the other said "check at desk". On a hunch we checked the passenger's submitted passport details, found that the expiration date was off by 1 day, and corrected it and were able to check in online and get a boarding pass as usual. Our guess was that a document-verification system had flagged the itinerary as not matching information the local government had on file for the traveler.