Yes that might be, I managed to fix the access to one of the customer accounts giving 404 error by making sure more customer groups were ticked - if only Guest or Visitor, then it gives 404 error, but as soon as I also ticked Customer, it was possible to log in again. The other one giving 404 works again when I clicked the button for converting Guest to Customer account. So seems to be some issue with the Visitor/Guest accounts, maybe some of my settings need to be changed.