Have just tried generating a 400-page ePub book, but get the error message "There was an error while attempting to render your book". Unfortunately there are no more details provided. Tried it in Firefox and Chrome. In both cases, it reached just over 73% while "generating epubfile" (having successfully completed Fetching and Rendering).
During Fetching, it reached about 90% before changing to Rendering. Could it have missed 10%?