Thank you so much jFiredog, it's "nice" to know I'm not alone. I can't spend time today messing with this problem but I will walk a little more lightly now I have an anchor. I'll report back.
UPDATE: Removal of KB3126593 worked for me too. Thank you to all who have contributed. I'm going to leave auto updates on and follow my normal backup routine. If the export fails then I'll know what to do.
A general question: from time to time I identify errors in a Microsoft product. In the current case it seems clear that there's something wrong with KB3126593. Is there any official way of informing Microsoft of the problem? And/or is there any way to dig into Microsoft to see if problems have been reported and what action is being taken? It seems that for many glitches there are plenty of reports of the problem with work-arounds but not much evidence of fixes.
I fear that Microsoft doesn't have robust User Acceptance Test and Regression Test procedures.
Thanks to all