Yesterday I met this error when tried to burn my WordPress blog feed at the feedburner.google.com:
“The URL does not appear to reference a valid XML file. We encountered the following problem: Error on line 6: The processing instruction target matching “[xX][mM][IL]” is not allowed”.
After checking feed page source code I discovered that it really has 6 blank rows before the first valid XML tag. So the problem is found but what is the source of such blank lines at the very beginning of the feed page? Thanks to the curious guys, after 30 minutes of Google reseaching I found these very useful for me posts: