An issue has been discovered with the 7.x-2.4 release of the contrib module "xmlsitemap" which may cause sites to go offline, with an error message similar to:
TypeError: Argument 1 passed to xmlsitemap_node_create_link() must be an instance of stdClass, boolean given, called in docroot/sites/all/modules/xmlsitemap/xmlsitemap_node/xmlsitemap_node.module on line 28 in xmlsitemap_node_create_link() (line 194 of docroot/sites/all/modules/xmlsitemap/xmlsitemap_node/xmlsitemap_node.module).This was reported on Drupal.org, and an updated version of the module has been released (7.x-2.5) which corrects this issue. Acquia recommends upgrading to the latest version of this module immediately, or disabling the module until you can do so.