This is just a quick tip. If you are getting this error message in Synaptic:

E: Unable to parse package file /var/lib/apt/extended_states (1)
E: _cache->open() failed, please report

or this error in apt-get in command line:

Reading package lists… Done
Building dependency tree
Reading state information… Error!
E: Unable to parse package file /var/lib/apt/extended_states (1)

and both just fail to open or install, then just execute this in command line (terminal):

sudo mv /var/lib/apt/extended_states /var/lib/apt/extended_states_tmp

You are done. Thanks to this post

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