When processing an HTTP request, there is a certain point when you can not redirect your process to an other page...
RedirectResult[
^] does exactly that redirection in the wrong place...
The reason for that error is that all/part of the HTTP headers already written to the output stream, and a redirection - being also a HTTP header line with 'location' in it - conflicts with existing header lines...
You probably have not to call base for OnActionExecuting, if you redirect the action...