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