[Plugin: Google XML Sitemaps] Problem Writing Sitemap Files

I notice my sitemap was not update on several months, so I try with the new version and then return to 2.7.1 but the error it’s always cant write on sitemap.xls and sitemap.xls.gz

Both files permissions are 777
Public_html permission 755 (I cant change it to 777 because a 500 error appears)

Please help, I really don’t know what to do.
Thanks