[roger64]

Thanks for your help

Well, on a third (!) examination, this is exactly my keyboard display (information coming from Debian tool). It gives in fact two different ways to type this damned ô as I wrote upper there.

The problem is that, using Sigil and only for Sigil, the highlighted touch (see red arrow) is totally dead: no way to type l'accent circonflexe or le tréma this way. As I never used other way to type them, I was at a loss and made do inserting them as special characters.

The reason for this is unknown. Well, sorry for giving all this trouble.