I am not sure if this will help... but I found that I had some code in a template
and I was starting with get_header() in this way:



    <?php
    /**
     * .. Template comments
     */

     get_header();

     if(...) {
        ...
        if(...) {
          ...
          wp_redirect($url);
          exit();
        }
     }
     ?>


and was getting the same issue of header previously sent... What I did was just move get_header() to the end of the block and voila!!!



    <?php
    /**
     * .. Template comments
     */


     if(...) {
        ...
        if(...) {
          ...
          wp_redirect($url);
          exit();
        }
     }
     get_header();
     ?>



No plugin was disabled. and everything was ok... you may give a try if this works for you