Shared photo streams suddenly have disappeared!

My mother suddenly had all the Photo Stream albums (which I had sent her invitations to and she had accepted) disappear from her iPad. Now when I send her a new invitation and she tries to click the link to accept, she gets the 'This photo stream no longer shared' message. She's talked to Apple support twice and they tell her the problem is with the sender, though I'm not doing anything different than what was done in the past and haven't deleted any of the photo streams. Support tells her that I have to turn on the "public view" setting. What gives?