Hello, I found a bug on this plugin. I know author suggested another place to report, but I didn't find out how to open a ticket there.
The bug happens when the blog url is changed, by editing wp_optons.siteurl and wp_optons.home
Doing that, everything else works fine on the new url (I do that a lot, I have a testing blog at home where I import my database and change its url), less Google XML Sitemaps.
Even I manually rebuilding sitemap.xml, it keeps pointing the xls to the old url location. If it is not accessible, I think Search Engines are still able to read the xml fine, but browsers render it all cracked.
I found out that the xls url is built using wp_options.sm_option, it remains with the old url.
To fix the bug, I just used phpMyAdmin to edit this row updating it to the new url, rebuilt sitemap.xml again and it worked again.
My suggestion to fix the bug is doing like everybody else and getting site url with php or wp_options siteurl/home (this can lead to another bug for ppl that use different values on them), instead of using a static value stored on a non-standard place.
Anyway, the plugin is great :D