When you get that error, it usually means you are outputting something before the html headers are sent. You may be using ‘echo’ or ‘print’ or some output function before the layout(s) and view(s) are rendered for the page you are receiving the errors on.
Thanks for this little info. I had a the same error message. With this info I was able to locate a single character has found its way somehow into one of the model file. After removing it all worked again.