All my wikis suddenly have the same shared user list

All my wikis suddenly have the same shared user list

I have a problem since this morning. After creating a new wiki I went to add some members. But navigating through Setting, Users all I get is a listing of 'Shared users' whom I can individually remove. Moreover, this list now seems to be common to all my wikis. When I naively started removing those I didn't want in the new wiki (i.e. nearly all of them) I discovered that I was removing them from all my wikis - specifically, from the ones they are supposed to be members of. Ouch! What is happening? My projects are unravelling...