synaptic does not work from Preferences (why there?) for (mere) user (does waste time) ¿why is it even listed!?
but using Debian/Applications/System/Package_Management brings up LXTerminal (did a message about gksu flash by?):

==== AUTHENTICATING FOR com.ubuntu.pkexec.synaptic ====
Authentication is required to run the Synaptic Package Manager.
Authenticating as:  root
Password:
==== AUTHENTICATION COMPLETE ====

messages repeated several times:

Gtk-message: Gtk-Dialog mapped without a transient parent. This is discouraged.

(also some messages about theme engine not found for (from-menu) app)

(on (possibly-buggered(never trust an "update/upgrade")) LXDE installed from Jessie DVD (only DVD I've seen))

(edit for format, specifics 2017JUN16)