Error message

• Warning: Cannot modify header information - headers already sent by (output started at /home/axipast2/public_html/includes/common.inc:2861) in drupal_send_headers() (line 1551 of /home/axipast2/public_html/includes/bootstrap.inc).
• Error: Class "DOMDocument" not found in filter_dom_load() (line 1107 of /home/axipast2/public_html/modules/filter/filter.module).