Same issue here, and I noticed it does not happen on all my category feeds, only the RSS2 and not all of them. I also checked all the things that you tried with nothing working.
As a temporary fix, try renaming the category slug, view the new category feed and see if it fixed it like for me. Then rename it back to what it was and well the renaming fix sticks and the blank line is gone.
Still trying to find a root cause.
