hey lionpeloux,
if you get the issue with the latest stable or development version of scribus this is to be considered a bug.
sadly, most advanced users have learned that using accents is to be avoided: at some time some software will have bug with it, so those bug get even harder to catch because many users never see them...
so, now we have the chance to catch this bug!
what is your operating system (i guess a version of windows...) and the scribus version?