Okay, I know you're aware of the problem, so I'll just add the errors I'm seeing.
Call to undefined function is_plugin_active_for_network() on line 271 That's the last thing the RSS feed will spit out.
The feed opens with
error on line 37 at column 1: Extra content at the end of the document I'm guessing that is the fatal error being spit out.
I also saw
parser error : CData section not finished which I believe is a direct result of an opening tag, but no closing tag due to the undefined function.