Feedburner email subscriber woes

[Tink]

I am subscribed myself to my own blog feed by email.  I have used two different services, gmail and comcast, for many years, so I can troubleshoot the differences if problems occur.

Well, a problem has occurred. I noticed last week that I stopped getting the email subscription, on BOTH email services, so I went to Feedburner to find out what was wrong.  I received lots of errors, none of which I wrote down, and after doing some research, I decided to resync.  When I did that, I received this error:

"The URL does not appear to reference a valid XML file. We encountered the following problem: Your feed has an invalid character(s) in it. This may be caused by editing your posts with a wordprocessing program. Try copying your posts into a plain text editor (like Notepad or TextEdit) before pasting them in your blogging tool."

Now, the odd thing about this is that the feed is showing up fine in Feedly, which is perhaps using the original feed for the blog and not the Feedburner feed.

My blog has over 6K posts, spanning some 5 years.  I can't even begin to figure out how I would find and kill the culprit.  I have read that this problem can be caused by using Word to compose, but I have never done that.  Any clues as to how to figure out what's wrong and resolve it, other than just ditching Feedburner for email subscribers?

thanks
Tink