Flying ex CMB to Honolulu -on OZ fare - CMB-HKG-ICN on CX no problem. Approached by supervisor in Seoul and told my paper ticket is not valid for C segment to SFO - and that I would need to produce 21 c-notes to continue on... I kindly protested - producing every sort of documentation necessary - to no avail. Forced to purchase $750 OW COACH to SFO under duress - in order to continue on. Advice?? Fare was OZ C fare. (BTW - fare now gone from system)