*** I started this thread then solved it but thought I would post anyway in case others are having the same issues - I didn't see any other threads/posts that mentioned this solution.
Symptoms:This happened in only one post, and it happened to be my most popular recipe this month, but people could not post a comment. I did have one comment under that post, which I used as a test comment, then deleted it. I have comments under other posts and have deleted and re-commented with no trouble.
http://www.cookswell.com and the post is Breaded Chicken Breasts.
I have the setting as comments must be approved, you do NOT have to register and login, you DO have to leave email and name.
I tried to click on the Post header to see if that would get me to the full post and let me comment from there. Got a 404 Error.
Checked my permalinks and remembered I had changed the way they were setup after I had posted that recipe. So I went to Manage>Posts and checked the permalink format and sure enough, it was the old method. Clicked on the little Edit link and fixed the back end of the permalink and all is fine now.