Kind of suspected that 3D Secure verification might be the problem alright. I've seen this happen before but then for many (most?) users it just works as you would expect. Don't know why it sometimes doesn't work. Or why 48 can't fix it so that it just works for everybody... Hopefully they can sort out out for you two sooner rather than later (although it's already later it seems!).
I am not a 48 employee so if you need to contact 48 support then use option 3 or the chat facility here.