@JeremyCherfas Dropbox has been known on occasion to mutilate the metadata in repos, rendering them corrupt. I think it has to do with it syncing changes as you're doing local changes yourself. But I've seen a lot of advice to just not keep repos in Dropbox - and then a lot of people who say they do it without problem, so, YMMV. :)
Doing a git pull should be faster than re-copying all the things all over again. It lets git do what it's good at: quickly pushing diffs around.
// @matigo