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. -- Thank you, David --
