No network after Trisquel 8 upgrade

I've just upgraded Trisquel from 7 to 8, and it seems to have gone horribly wrong.

Firstly, X doesn't start automatically. However, I can do Ctrl-Alt-F1 and log in,
then do 'startx', and it brings up LXDE, so at least X does work.

Secondly, I can no longer access the network; when I do 'sudo ifup eth0' I get:
Internet Systems Consortium DHCP Client 4.2.4
Copyright 2004-2012 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/
Usage: dhclient [-4|-6] [-SNTP1dvrx] [-nw] [-p ] [-D LL|LLT]
[-s server-addr] [-cf config-file] [-lf lease-file]
[-pf pid-file] [--no-pid] [-e VAR=val]
[-sf script-file] [interface]
Failed to bring up eth0.

I have no clue what that means.

Thirdly, and worst, even if I could get the network up, apt-get fails.
When I do 'apt-get --version' (or any other parameters, or none at all) I get this:

apt-get: relocation error: /usr/lib/x86_64-linux-gnu/libapt-pkg.so.5.0: symbol _ZNKSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEE7compareERKS4_, version GLIBCXX_3.4.21 not defined in file libstdc++.so.6 with link time reference


Similar error messages (different libraries, different symbols) occur when I try to run various other programs.

Aargghh! Somebody, please help! :)

Obviously something(s) vital has been incorrectly upgraded, but I've no idea how to track it down.

I can boot Trisquel 8 (older alpha version) from a USB flash drive, if that's of any use.