No plugin is active. This happens also on old posts but just when I go and just “save and continue” or “save” them. The markup remains unaltered until I go through admin controls to edit and save the post. Any other markup tag than p or br is cleaned (eg: img, blockquote, ol…).
For now, I’m working with an offline client but my other users are pretty upset by this.
In addition: I must say this is not happening in other wordpress blogs (ie: trial blogs, my local install…)