Error message

• Warning: Cannot modify header information - headers already sent by (output started at /var/www/www.edv-bartl.at/docs/includes/common.inc:2748) in drupal_send_headers() (line 1233 of /var/www/www.edv-bartl.at/docs/includes/bootstrap.inc).
• Error: Class 'XMLWriter' not found in require_once() (line 15 of /var/www/www.edv-bartl.at/docs/sites/all/modules/xmlsitemap/xmlsitemap.xmlsitemap.inc).