I read a similar post, bit unfortunatelly without conclusion.
I have Problems login into MyBook Live. When I enter the correct password I returns me to the login page without an error Message. If I try to enter a wrong password, it tells me the password is wrong. Thats the only difference.
I can login via ssh. There I see the root filesystem is full.
Try to reset unit and see if you get the same results. Lets see if another user that can help by sharing some tips or information on this matter can assist.
Same exact situation here.Hope we find answer. Read reinstall of firmware could fix issue. A manual install of this firmware can be done via ssh but I am not yet up on the procedure to do so. At least we pre enabled ssh otherwise may be no solution. If you try an apt-get update you get told not enough space???