mrtz:
For the problem below.
I think it's about permission. So, I did the following steps to make it work :
Look for file "/includes/modules/pages/sitemapxml/header_php.php"
Search for : $zen_SiteMapXML->setStylesheet($tpl_dir . '/gss.xsl');
Then, change to : $zen_SiteMapXML->setStylesheet('/gss.xsl');
Then, copy gss.xsl file from /includes/templates/template_default/css/gss.xsl to /gss.xsl
Generate Sitemap again. By the way, I'm using Zencart 1.3.9c
Hopes this helps: I upgraded to 1.3.9b and started receiving all sorts errors with this add-on, which I had working fine earlier. The above mod does work, for the "trouble loading xslt stylesheet error. However, I had to manually edit the individual sitemap file (sitemapproducts.xml, sitemapcategories.xml, etc) to point the stylesheet href to the new location. Since these files didn't change, they were not updated when I reran the generate Sitemap tool.
I also received an error: "junk after...<url>^ at line xxx in the Sitemap.xml file. Once I found line 439 (about halfway through file)while editing that file, I found a </urlset> stuck in the middle of the sitemap. I moved that line to end of file and this eliminated that error.
Now, all of the sitemaps appear to be working again.