Just found 2 blank lines at the end of my theme’s function.php file. My RSS feed is now working properly.
I’m actually experiencing the same problem on my blog: http://mrwebwriter.com/
I pinpointed the problem to the current theme I’m using.
I can see in the source code of the RSS file that there is in fact a blank line at the top of the document. My problem is that I’m not sure how to remove that blank line.