I fixed the feed. At least this link, originally supplied by Jukka, says we check out.
There were two breaks. The first, noted in the comments to the answer by Rebecca Chernoff, was a form feed that appeared in a post of mine. The other break appeared in an earlier post by Artem (on the negative adversary method). In both cases, I erased and retyped into the blog, in the HTML window, the characters that appeared there. (One of Artem's references was producing an error.)
So I'm not sure how to avoid this in the future, but things seem repaired for now.
EDIT: We now appear on blogoverflow, but not on the TOC Aggregator.