RSS 404 error

I’m getting a 404 error for my RSS feed. (https://clearaction.biz/blog) — I’m sure the solution must be rather simple but I’ve spent hours and hours upgrading everything as re-set. I’d be most grateful if you could guide me!

Here are the facts of my situation:
A) no problem getting RSS feed before 9/27/08 post (i.e. feed exists up till then on my profile page: https://www.thecustomercarenetwork.com/profile/LynnHunsaker)

B) got my WordPress account via my Yahoo!Small Business Merchant Starter Account

C) ugraded today to WordPress 2.6.3 and to latest version of my WordPress theme

D) installed today FD Feedburner Plug-in: not sure what to type in the plug-in feedburner configuration form (redirect my feeds here: _____, redirect my comments here: _____; instructions say to first go to Feedburner and then enter the URL it created for me)

E) Feedburner gives me an error when I type in https://clearaction.biz/blog/feed/ (says it doesn’t appear to be valid feed due to an error on line 2: the processing instruction target matching xml is not allowed)

F) removed 2nd line in my theme editor header.php file — same error as in (E) above — what file is Feedburner referring to?!!!

G) tried https://validator.w3.org/feed/ and got this message: Server returned HTTP error 404

H) google help suggests that RSS feeds sometimes quit working if <:-0> code exists — can’t see anything like that! — or if a blog post originated in MS-Word — in fact, the 9/27/08 did originate in MS-Word, so I copied the post’s text from the screen, and re-pasted it into the post — but no changes with Feedburner or Validator :-0

I’d be most grateful if you could give me some pointers!

Thanks,
leren