Yes, forgot to say it ... I also tried the combinations : the same preamble for the master and the incriminated file, only the file, only the master.
Do you think i need the preamble for 'each' subfile ?
_all_ LyX files must have the _same_ documentclass and the preamble. Then you can edit and run every subfile as its own and all packages should work in a proper way.
Herbert
