Fix my Wordpress v4.5 - error messages that does not go away, prevents publishing workflow to complete

We are getting this error: [Mon Aug 29 10:07:44.060509 2016] [:error] [pid 1167] [client [login to view URL]] PHP Fatal error: Call to a member function getHeaderValue() on a non-object in /var/www/mightytravels/wp-includes/[login to view URL] on line 635, referer: [login to view URL] [Mon Aug 29 12:16:03.864020 2016] [:error] [pid 3944] [client [login to view URL]] PHP Fatal error: Call to a member function getHeaderValue() on a non-object in /var/www/mightytravels/wp-includes/[login to view URL] on line 635, referer: [login to view URL] [Tue Aug 30 05:21:05.915987 2016] [:error] [pid 27309] [client [login to view URL]] PHP Fatal error: Call to a member function getHeaderValue() on a non-object in /var/www/mightytravels/wp-includes/[login to view URL] on line 635 ubuntu@newmt: ~/mightytravels $ This may or may not be caused by a plugin (but we have them all in the newest version and we REALLY NEED them). We will provide the [login to view URL] on our system (it is not modified) on request for you to fix. We are running Ubuntu and PHP 5.5.9-1ubuntu4.19. Thanks!