I seem to have the exact same symptoms as How to fix user avatar? but I’m running the latest version of discourse (I tried to upgrade to fix this).
Upload works fine (file are being uploaded to the shared folder) but when the users save his profile, it goes back to the default avatar. It’s the same for gravatar.
Ok I solved it.
It was due to the forum being in a subsubfolder ie /forums/foo, I followed this guide to set up the forum but it was missing a symbolic link to reflect this subpath.