The exception is basically telling you, that PrettyFaces tries to forward the request from the virtual pretty one (for example “/login”) to the real one (for example “/login.xhtml”). This fails because the response has already been committed, which means that someone already wrote something to the response.
How did you map the FacesServlet? The view-id has to be the same URL you typically enter into the browser bar. Something like /faces/login.xhtml or /login.jsf.
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