I run Standalone Sitemap Generator (PHP) v6.0 which creates site map.html ror.xml and urllist.txt OK but the sitemap.xml is always empty.
I have spent a few hours looking at all the documented issues and cannot find any of the errors, in many cases an admin has looked at the problem but no there was no follow up information in the forum.
Anyway, I found the problem in the end by looking in the apache error.log file. Somehow the sitemap_xml_tpl.xml had an extra "dot" on the end - must be accidentally self inflicted.
Looking in the web server error log is somewhere to explore in future.
Hope this helps someone else