Unfortunately I don't recall if I updated any plugins. It's quite possible that I did, but didn't take much notice of what ones.
I haven't installed any new plugins or themes. Or updated the theme.
I did disable the browser cache on the W3 Total Cache plugin, because of another issue I was having. And actually today I disabled W3 Total Cache to see if the browser cache change was affecting the Calendar, but there was no change.
And I set up a cron job to trigger wp-cron.php regularly.
There are no errors showing on the Calendar page. Just a lot of blank boxes which should have my posts in them.
... Out of curiosity I just added a new draft post via the Calendar. I can actually see that one sitting there all on its lonesome now.
... Oh wait... hang on a minute. I just logged out and logged back in again, and now I can see *Some* of the posts I should be able to see, but not all of them. There is a block of days with all their posts, and the other days have none of their posts. There are none with only some posts
... then I clicked on Posts, then back on Calendar, and all the ones that reappeared have disappeared again :o
But the new one I added via the Calendar still shows up.
... but then I click on that one, and drag it to a different day, click on Posts then back on Calendar, and it's disappeared again.
There are no obvious error messages showing on the page
Strange things are afoot!!