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In the middle of this I had changed from Jessie (testing) to unstable and being "unstable" had some problems logging into Gnome-Shell afterwards. Evidently I had broken packages - hell it was a mess, let's not mince words!
Somehow Synaptic had selected "Debian" as the default repo during this, and as I cleaned up my sources list, didn't notice this - there is no sources list called Debian.
So, next time launching Synaptic it complained about 'Debian' being invalid and hence the error message in this article title!
So, like any reasonable Linux user faced with problems, I Googled for a solution. Well, none of the solutions worked for me - probably because they weren't Debian and/or Synaptic specific.
To cut to the chase here is where Synaptic stores the default release setting: '/root/.synaptic/synaptic.conf'. Hope this helps someone else!
Guess the moral of this story is not to rely on a package manager and use 'apt-get' and/or 'aptitude' from the command line - which I usually do. What can I say, me likes to try new things once in awhile. lol