Sorry for replying to my own message so hastily, but making further tests I may have found something
interesting:
I have many beams going over rests. If I remove every manual beaming over rests, the problem disappears
and everything works fine (except for the fact that beaming over rests is important information in my music).
It looks like some bug that breaks the french-beaming functionality when beams extend over rests. I tried
disabling the stemlets to see if they had anything to do with this issue, but the segfault persisted.
Best regards!