Editor>Preview Starts Session-Expired Popup Deathloop

Just like it says in the title…when I’m in WP’s editor working on a post, everything seems fine until I hit Preview. At that point, it opens up a separate window and also helpfully starts the notorious WP Session-Expired Popup Deathloop.

I know that this curse frequently goes back to having different entries in the “WordPress Address (URL)” and “Site Address (URL)” fields, but mine are and always have been the same.

I also know that reasons for this misery are often never discovered, but in that I have a specific, repeatable trigger for this well-known problem, I thought we might have a chance of finding a quick solution.

Any helpful thoughts will be appreciated.