thanks for your answers
The fanfics have been transfered in a kindle without any problem and are very famous (Harry Potter fanfics by Alixe), so I think that if there were a problem with them, someone would have noticed... But epubcheck is a good advice, I didn't knew it
Sometimes an epub is "strange", I am happy for having find this software thanks to you !
And now I hesitate to test with another PC because everything is ok for now, and I don't want to break everything
Oh ! I will try with my wife's Kobo ! lol (joke)