The supplier of my virtual server say they can't see anything wrong.
xml-sitemaps ran happily for several years before failing to log a useful last modified date/time.
Getting this as a header report:
Response Code HTTP/1.1 200 OK
Date Tue, 03 Jan 2017 18:20:31 GMT
Server Apache/2.2.31 (Unix) mod_ssl/2.2.31 OpenSSL/1.0.1e-fips mod_bwlimited/1.4
Accept-Ranges bytes
X-Mod-Pagespeed 1.11.33.4-0
Vary Accept-Encoding,User-Agent
Content-Encoding gzip
Cache-Control max-age=0, no-cache
Content-Length 6661
Connection close
Content-Type text/html; charset=utf-8
Don't know what to do next:)