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 2: We now appear on blogoverflow, and the TOC Aggregator. The traffic is picking up.
BUMP for EDIT 3: New post up, it took about an hour for it to appear on blogoverflow and for the flag to appear on CSTheory. Those are now in place, but it is not on the TOC Aggregator. The feed checks out valid.