I looked for this via search but could find anything. I hate to think I'm the only person to have screwed this up....
Installed Post indexer and then Recent Global Posts Feed to create a feed from the site. This is WPMU 3.1 and BP 1.2.8
I get this error from the feed:
error on line 2 at column 6: XML declaration allowed only at the start of the document
The widget file is in mu-plugins and the primary php file is in wp-content.
Here's a screenshot: