I had the same problem last week and kept receiving text messages and emails telling me to complete API, even after I had checked in and had my boarding pass for LHR-JFK, although I had to do a doc check at LHR. GGL CS advised me that all fields had been completed correctly and that I should do the doc check at the check-in area rather than at the gate. Document check took a few seconds; all was OK.

Doc Copper