This could be very much a machine specific issue, as this one has form. Just before submitting an edit to a post, Windows Explorer kicked the pail and restarted, and the Edge tabbed windows assumed a different order. Nothing in the system logs for that, so no way to ever determine if these things could ever be related.
After submitting the post edit, the page content refreshed as expected, except for one small detail, the additions to the post were not there! On reloading the page, horrors upon horrors, the login dialog showed instead, and naturally the extra edit content was lost for good.
Currently, we are not running WP in debug mode,… in the meantime this, and other issues may go away once on board the new provider. 🙂