Had same problem, apparently caused by a line in the pre-update robots.txt file.
Solved for me by deleting the ".gz" at the end of that Sitemap line in the robots.txt file, then resubmitting the sitemap.
So, instead of "Sitemap: http://yourdomain.com/sitemap.xml.gz", the line is now , "Sitemap: http://yourdomain.com/sitemap.xml"