Often a case of starting a session when something is sent to browser. Even a space can do this. But with all due respect you are asking a question that might have a multiple of causes but give no information such as what program causes it or a url someone can try with diagnostic software running!!
Tried to edit above but couldn't. I meant to say: Often a case of starting a session when something HAS PREVIOUSLY BEEN sent to browser. Even a space can do this.
@thehpmc mentioned correct reason of this error but to investigate where is exactly this output is began we need more info. Please contact us in helpdesk and we will help you with it.