Arantor,
Thank you very much. I would never have found it by myself, so I switch right on and will retry again. I also noticed that the copy/paste feature wasn't available either sometimes. I'll check that one too.
PS: this problem is solved but surprise ! Now without WYSIWIG the background of the editor field is black in the CC 2 theme, which is the default and the one used by most actual members... I'll have to edit the CSS... :/