polls.html works, but polls.xml fails

I reproduced (part) a problem of mine using the excellent Redmine Plug-Tutorial.

I added a plugins/polls/app/views/polls/index.xml.erb.
http://localhost:3000/polls.html?project_id=test works fine, but when I call http://localhost:3000/polls.xml?project_id=test I in the server-log

Filter chain halted as :authorize rendered or redirected
Completed 401 Unauthorized in 3.8ms (ActiveRecord: 0.5ms)

Why do I get this error? I thought that rails renders xml or html.

Thanks in advance for any hint.

Redmine