My URL value in config.json was : "url": "http://forum.xxx.fr/"
I deleted the "/" at the end and everything went back to normal.
Don't know how this "/" appears.
Right it's fixed 🙂
@pichalite that was it. It's a bit confusing... Maybe the private option for specific groups should be grayed out if the general option for allowing private groups is disabled, to make this clearer.