Looks like a problem with 3.1. Same thing is happening for me and I don’t use Simple Tags.
A bunch of others (including myself) having the same problem. I had to revert to my backup and the WP3 install. This is really bad and I hope it gets fixed soon.
@muchacho79 – Custom or stock WP theme?
I have the following option checked in Yoast WordPress SEO: “Check this box to enable XML sitemap functionality.”
However, I cannot locate any file named sitemap.xml anywhere on my hosted server. Where does this file actually exist? Now that I’m linked with Google Webmaster Tools, anything more I need do to actually create/link it properly?