Comment 5 for bug 1717536

With a wayland session (now default), all you need is :
- open a terminal
- then copy/paste & execute: xhost +si:localuser:root

then you will be able to use synaptic

If you open a X11 session (i use lightdm here), then no problem.

note: if wayland/xwayland has been activated a while back, that issue has only appeared with the 3.25.92 'gnome & the likes' version