According to the W3C, your RSS feed looks good:
The error is not showing up anymore, but if you read it carefully, you'll notice that the <?xml... of the feed declaration was on line 2. Now it is at the start of the document, where it should be. That means you've added some additional content to the response. Maybe some unintentional whitespace. This could even happen with plugins generating unwanted content or you making modifications to files without noticing you were generating unwanted content.
I'd say the issue is fixed but be careful with modifying files and make sure to track those changes next time to simplify fixing the issues in the future.
Note if you still see the issue, then there must be a condition tied to your specific user or to logged in users which is not seen by anonymous users, like me :)