I was just having this issue on a straight LHR-HKG return with a US passport. I think it may have to do with the length of time I have spent in HK recently or will spend in HK before my return flight (hence requiring a visa, which luckily I have.) That said, it wouldn't accept my visa info, possibly as the only "number" I had for my evisa was just the "application reference number." What worked was inputting an "alien card" instead of a "visa" as the additional documentation required and using the number for my HKID (only worked also without the parentheses), but since HKIDs don't have an expiration date, using the expiration date of the visa.