Hi
I have just had the same issue and the solution was to enter via the 'Manage' top left of screen. Apparently system doesn't work if you enter other ways. Then enter your six digit booking code and surname.
I entered my partners details first and it worked. I then entered mine and it didn't. I then shut the page down and returned via 'Manage', booking code and surname. Partners had saved and I entered my details again and saved it.
Hope info was useful and good luck.
Ian