"This error also presents as "Cannot modify header information" depending on PHP version.
In short, it means that somewhere in the code, something was printed to the browser before Drupal had finished preparing the page. This is most often caused by custom or modified code contributed from sources outside Drupal, so inspect your uniquely added code (including themes) first..
If you get a "Headers already sent" error, there are three likely causes.
If this error is not the first error message on the page, then it is most likely a 'avalanche effect' of previous errors and you may ignore it. Instead, focus on fixing the errors before it. When you fix the first error message(s), the "Headers already sent" error(s) will most likely disappear."