As I expect the layout problem was solved with a message posted on the net. I did it on Google Groups, in the area for bloggers. Luckily, a day before someone with the same problem posted on the group and has already been answered.
The answer came from Most Frequent Blogger Questions (a really nice reference for whom uses Blogger). I had to find which of my old posts were the bad one and it was easy: I changed the number of posts per page until find it.
It's done and my job is over. Once more, even we don't realize, the flat world, shows up.