put something into “List of IP addresses and networks that are allowed without auth”
?
If you did, revert that
Either of the above effectively disables user authentication, so that anyone’s activity within your local network can get attributed to the server owner’s account (i.e. you).
Change that to cover only a few select IP adresses, which should of course be fixed IPs of your own devices which are not (regularly) used by your family members.
I’ve left DLNA unchecked and cleared the field concerning auth devices for now.
However neither of these things relate to why this issue has only just started recently. I’ve never had DLNA on and always had 192.168.1.0/24 in the network auth field.
Authentication of a client device will automatically be made invalid every few months for security reasons.
Maybe this happened on the device which is used by your son. Or maybe it was removed manually from your account: https://app.plex.tv/desktop/#!/settings/devices/all
Without the above setting in “List of IP addresses …”, the device would have simply lost access. But with the list being present, it will silently gain access to your server, since authentication is not enforced.