Same problem here. I've checked the right, they're ok, I've emailed my host (GoDaddy) and here is answer - seems to me they have no idea, what I want them to do. I have access to php.ini, would that be helpful?
My host response:
Thank you for contacting Online Support. It is my understanding you are having trouble with your website. This error is caused if the your PHP scripts are printing to the browser prior to sending headers. A common example is printing the html tags prior to starting a session, or setting a cookie. The error tells the line that needs to be altered. To resolve this error remove the lines from the PHP code that are printing to the browser prior to sending headers.
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