AC-66U B1 - Unable to set-up Guest Network

Hi,
When I try to configure the parameters for a guest network, whatever I do, when I click on "Apply", new fields do appear (Key index, WEP Key 1 to 5, ASUS pass phrase).
And from that moment on, any new click on "Apply" just does nothing at all !

Any idea ?

Thanks for your help and have a nice day

EDIT: I tried with Firefox and Chrome. Same problem with both