[stumped]

just for the record, and to close this off. I have made several more changes to the book I was working on ( it is a purchased epub game guide from gamepressure.com ) correcting bad English and removing unnecessary bits . So I have several times deleted it from tablet and from CC's records and re-fetched it from content server after editing it on PC. That has worked every time. I am unable to replicate the cannot download format error.

also for the record, the issue of content server serving a cached copy still happens.
i.e. if you have sigil as your default epub editor, double click a book title in calibre main window to edit it, and save it from sigil after editing [ which returns it to its original place in the library ] - after that has completed, the content server gives out the previous version, [ which no longer exists in the library because it was overwritten by sigil a few minutes earlier. this continues to happen until calibre is fully closed and then restarted.