this seems to be 1.5 problem. some users claim that replacing the offending feed templates will fix it (hasn't worked for me). others claim some plugins are at fault (not for me: I turned them all off to no avail).
What's the fix for a feed that fails to validate due to an empty first line.
line 2, column 0: XML parsing error: <unknown>:2:0: xml declaration not at start of external entity