The "RSS element" formatter doesn't work as expected when using stream wrappers to handle files (e.g. when using the Storage API module without any local file storage).
views_rss_core_field_formatter_view(), the use of
drupal_realpath() means that the formatter tries to generate a derivative on every request, which results in a database duplicate key error.
As far as I can tell, it should be possible to get rid of the call to
drupal_realpath() since the functions using $real_path all support stream wrappers.