Very strange. Two of my blogs (so far...) have suddenly closed all comments on all posts. One of the two blogs has two posts with open comments, the two posts published yesterday. The other blog hasn't been updated since Sep 17 and the same thing happened there.
In addition to having comments on EVERY blog post (except the two listed above) deselected on EACH blog page, there are other weird issues. Three discussion setting that I have never used, never set, and certainly did not change recently were checked today on BOTH blogs.
(1) Users must be registered and logged in to comment
(2) An administrator must always approve the comment
(3) Break comments into pages with...
Strangely, neither of them had the "Automatically close comments on articles older than x days" -- even though that is what seems to have happened.
Tried deactivating EVERY plugin - no change. Tried changing to default theme - no change.
It seems I can change these manually -- although I don't know if they will eventually revert -- but I have hundreds of posts.