Hi @websiteowner,
this warning often comes when there is something before the header. This is absolutely forbidden, including a blank space which is not actually nothing, but a character.
Sometimes comes about if stuff is copied and pasted or edited in a word processor app rather than plain text editor as word processors add invisible formatting characters like the new line character for example '\n'. Plain text editors don't do this.
Anything before the header, is seen as the header........and it sees the real header as an attempt to modify the header, which as you know can't be done once the header has been sent. 😊
