go to your plugins folder and begin to open every .php file one by one.
then: scroll down each file down to the bottom until you encounter "?>" without the quotes.
example php file:
text... code ..
some more code...
bla bla ..
look if there is any "space" right AFTER or BELOW the "?>" or if there is even a "return" (new line).
Delete any white-space after this and save the php. Any php/plugin file may not end with a empty line or space!
Note: Backup all your plugins before you do that!
I did that with all my files and then my feed was working again.
Edit: and dont delete any text or code.. just the last line of the file needs to be checked for empty spaces... and if there is no ?> on the end of the file then move on to the next.