Each time the RSS feed is created / updated in Media Gallery, it attempts to set the file permissions on the RSS file. This is done because some sites were having problems with the permissions being set incorrectly which caused problems later.
On your site, the chmod (changing the file permissions) is failing probably because you either have safe_mode enabled or some type of other restriction in PHP preventing it from working.
The simplest method to solve this is to edit mediagallery/maint/rssfeed.php and on line 223 change it to look like this:
Text Formatted Code
@chmod($_MG_CONF['path_html'] . 'rss/' . $_MG_CONF['rss_feed_name'] . '.rss', 0664);
Also, in the same file around line 156 there is another chmod call, change it to look like this: