There was a previous thread for this but that did not really supply a solution since the solution was to disable the plugin :)
I've tracked this down to the function wprss_load_textdomain:
- load_plugin_textdomain( 'wprss', false, plugin_basename( __FILE__ ) . '/languages/' );
+ load_plugin_textdomain( 'wprss', false, plugin_dir_path( __FILE__ ) . '/languages/' );
The problem was/is that the plugin_basename will return wp-rss-aggregator/wp-rss-aggregator.php while we really just need the path.
See the above diff for a patch.