it is a sad thing that they are following the likes of dmm/fanza.
i contacted sokmil support, and this is their reply.
(translated meaning: since 4 august 1600hrs, there are issues with credit card settlement, and hence registration or payment cannot be done via credit card.
resolution is going on but there is no expected timeline of recovery.)
i reckon even if it is recovered, probably only japanese cards will be supported.
the support suggested to top up and pay via bitcash, but if you do not have a japanese address and phone number, you cannot top up bitcash via credit card. so you gotta purchase a bitcash prepaid card from another 3rd party vendor to do so, which brings even more hassle and risk.
i thought japanese is advanced in technology, but seems like they are going backwards in terms of digital payments.