I tried multiple credit cards from different banks and all returned with a cryptic "error". These cards are confirmed working as i used them frequently including for online purchases.
I just payed using Visa. I waited until the page said "Transaction approved". I then checked my credit card and a reservation from ISC2 was visible.
The ISC2 page kept on telling me that the transaction was approved so I refreshed (which i was not supposed to do until the payment was approved).
Now, according to ISC2, I still need to pay my annual fee. Even after I logged out and logged back in again.
I'll give it another day but needless to say this part of the website requires some tweaking 😋