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I'm pretty sure that happens because your session cookie times out before you submit the chapter, so some necessary information gets lost. Session cookies last 10-15 minutes from your last site request.
Backups and previews are good fail-safe practices but don't address the cause of the problem. Basically anything you can do to keep your session cookie alive should keep that situation from happening. For example:
Keep the Messenger pop-up window open while you're composing the chapter. It self-refreshes once a minute, so your session cookie gets updated.
Every five minutes, open or refresh another WDC page in another tab. This also updates your session cookie. If you forget and it's been longer than 10-15 minutes, you can do this to recreate the session cookie, and then submit the chapter add request. At least, I've had this work in other similar situations it happens, such as reviewing and adding Book entries.
Hope that helps.