It's a BUG:-

https://bugs.debian.org/cgi-bin/bugr...cgi?bug=867967

If I removed /etc/apt/trusted.gpg Synaptic launches w/o error but the next time Synaptic is launched the error returns and the
/etc/apt/trusted.gpg is re-created.

If I find a way to fix it by doing some googling I'll post the fix.

YES, I did try the other links:-

Since removing that file there hasn't been any more errors from Synaptic so far.

When I did the fresh install, I could not launch synaptic graphically (through GUI). A fix to this that I found is by installing the following packages:

sudo apt install synaptic apt-xapian-index policykit-1-gnome

`policykit-1-gnome` makes the GUI auth dialog for synaptic possible, without which synaptic will run, but it won't show anything on the GUI.

`apt-xapian-index` is basically for maintaining the package indexes.

I don't think it is a library issue but a policy kit issue and since different desktops use different ones synaptic-pkexec works on some and not on others.
Debian tried to rid itself of gksu/gksudo claiming it was abandoned and therefore insecure, so everything that may need rights escalation would be handled by systemd and -pkexec. Mainly synaptic and gparted are affected, maybe a root terminal.
$ sudo synaptic or sudo gparted should start the programs with or without policykit if the user is in sudo group and sudoers file is modified appropriately. If they don't work it is a program related issue, if they do, it is a policykit problem.

Automatic menu can create entries for pkexec files but not add sudo or gksudo in front of the program entry.