Happened again today after successfully submitting a post to another thread, so no logout issues. This time, hitting the back button revealed no post content. Must have been too quick for AutoSave, (or something) – changed the interval to 45 seconds.
Edit: Most likely related to an oops in the Cache Enabler Settings. Cached pages now expire 10 hours after being created, before they never expired, so could have been sending expired nonce data to the WordPress engine. The site cache was also cleared when a new post was submitted, now turned off. Cache is now turned on for mobile devices. 🙂