Error message

• Warning: Cannot modify header information - headers already sent by (output started at /home/customer/www/thanasisvirvilis.gr/public_html/includes/common.inc:2800) in drupal_send_headers() (line 1501 of /home/customer/www/thanasisvirvilis.gr/public_html/includes/bootstrap.inc).
• Error: Class 'DrupalQueue' not found in _update_create_fetch_task() (line 250 of /home/customer/www/thanasisvirvilis.gr/public_html/modules/update/update.fetch.inc).