This usually means you have a blank line of code, or content before the redirect.
So the page: by (output started at /var/www/vhosts/xxxxxxxxxxxxxxxxxx/line xx
that is where the line of code you referenced. This error should reference two lines of code. The code where the error occurred, and the line where the output started. Usually the correction is made in the line where the output started... if you send both lines then I could suggest which to update and to what.