after performing an upgrade from debian squeeze to wheezy I was unable to start
synaptic getting the following error messages:
"E:The value 'lenny-backports' is invalid for APT::Default-Release as such a
release is not available in the sources"
"E: _cache->open() failed, please report"

after checking the configuration files in /etc/apt/apt.conf.d and preferences.d
I could not locate any entry having "APT::Default-Release" set to lenny
backports. The problem persisted even after completly purging (apt-get purge)
synaptic and reinstalling it. Finally, after checking synaptic's configuration
file in /root/.synaptic/synaptic.conf I encounted the line DefaultDistro
"lenny-backports". After changing its value to "stable" synaptic started
without any problems (removing the configuration file works too). However I
assume this is related to my system which has been been upgraded since Debian
Etch while using lenny backports for some packages. Beside the problem with the
(obviously old) configuration file I assumed purging the package would remove
the faulty configuration file.

Purging a package never removes files from a users HOME.

I had a similar problem with synaptic 0.81.1ubuntu1 after doing an ubuntu
upgrade from 12.04 Precise to 14.04.1 Trusty

E: The value 'stable' is invalid for APT::Default-Release as such a release
is not available in the sources
E: _cache->open() failed, please report.

Removing the line that mentioned "stable" from
/root/.synaptic/synaptic.conf allowed synaptic to run without problems.