Hi Ladan, there may be conflict with "Google XML Sitemaps" by Arne Brachhold that you have enabled. I would start first by disabling that plugin to see if the issue is resolved.
You also have to rename sitemap.xml in your root folder to something other than "sitemap". This file was created by Arne's plugin and is no longer needed if you have XML Sitemaps enabled in WP-SEO.
At line 344, Yoast's plugin installs a query for "sitemap" and the combination of the two may result in the issue you are seeing.
You can also disable XML Sitemaps from the WP-SEO admin panel to prevent line 344 from ever executing.