[eluser]tonanbarbarian[/eluser]
Cannot modify headers usually means that something has been already output
One of the common mistakes is blank spaces after the closing ?> tags in code files
In fact is is better just not to use closing ?> tags in files that are only code (i.e. Controller, Models, Libraries, Plugins and Helpers)
Or if you have added some debugging to your code to echo out something then it may be causing this issue as well