Overriding from rss.php has no effect?

I tried to overwrite the file (wp-content/themes/[mytheme]/plugins/events-manager/templates/)ical.php. But unfortunately without effect. My changes were not displayed when I open “feed://www.mysite.de/events/feed/”.
I see only the unchanged output.
The overriding from ical.php in the same folder (wp-content/themes/[mytheme]/plugins/events-manager/templates/ical.php) works.
What am I doing wrong?

http://wordpress.org/extend/plugins/events-manager/