Un point d'exclamation s'est installé dans la racine de deux de mes dossier.

Two things: I don't work for Adobe, I'm a Lightroom Classic user just like you. That means I have no obligation to give an answer to your reply on my first question. I can choose to answer if I think I have something useful to contribute, or I can choose not to.

Secondly, no I did not have 5 minutes to read that reply. I've been away for two days and I've not been in the forums in those two days.

And finally: you still did not give me the information I'm looking for. You say "If I place new images in an existing folder, it’s impossible for me to “synchronize” them", but what does that mean? What does (not) happen when you try? Do you get the first dialog that tells you how many new images there are, but does Lightroom not import them when you click on import? Do you not get that first dialog at all? Do you get the first dialog but Lightroom does not show any new images? All this matters when trying to find a cause.