[ForceHSS]

Disable all plugins then enable each one to find the problem one might just need an update to it
Or
There are few things which can produce this warning :
If you are using cookie-based sessions, you must call session_start()before anything is outputted to the browser.
Remember that header() must be called before any actual output is sent, Either by normal HTML tags, blank lines in a file, or from PHP. It is a very common error to read code with include(), or require(), functions, or Another file access function, and have spaces or empty lines that are output before header() is called. The same problem exists when using a single PHP/HTML file. - Or maybe the problem is about the corrupted characters before <?php . Those appear when you changed encoding ANSII to UTF-8. Edit your configuration.php with a HEX editor or re-encode it with Notepad++ and select "Encode in UTF-8 without BOM".
http://bytes.com/topic/php/insights/...s-already-sent