I have had this same thing happen when flying to Costa Rica on a one-way ticket as a USA passport holder. In that case the system recognized CR's rule that I must have an onward ticket, and would not let me check in until I got to the airport and showed the agent my onward one way ticket (also on United, interestingly, but the app didn't pick up on that). Why this showed up as a passport error, who knows? IT issues, I guess.