Thank you for your reply. The good news is that the problem described has gone away. Based upon your suggestion, I deactivated the XML sitemap plugin, did a preview (successfully), and then reactivated. That might have triggered a reset if that was the problem. Not sure since I did not check beforehand to see if it was still happening. In retrospect I should have, so the exact cause is still uncertain. Might have just worked itself out overnight.
