Comment 24 for bug 1354924

SOLVED!!!

The hint to the solution was the content of networking.log:

run-parts: failed to exec /etc/network/if-up.d/static-routes: Exec format error
run-parts: /etc/network/if-up.d/static-routes exited with return code 1
Failed to bring up lo.
run-parts: failed to exec /etc/network/if-up.d/static-routes: Exec format error
run-parts: /etc/network/if-up.d/static-routes exited with return code 1
ifup: post-up script failed.

So I compared the /etc/network/if-up.d directories on my working 32-bit and 64-bit laptops. The difference was a single executable file on the 64-bit affected laptop but not on the unaffected 32-bit laptop:

-rwxr-xr-x 1 root root 351 Oct 17 16:25 static-routes

Which contains:

ip route add unreachable 10.0.0.0/8 metric 999
ip route add unreachable 172.16.0.0/12 metric 999
ip route add unreachable 192.168.0.0/16 metric 999
ip route add unreachable 169.254.0.0/16 metric 999
ip route add unreachable 192.0.2.0/24 metric 999
ip route add unreachable 198.51.100.0/24 metric 999
ip route add unreachable 203.0.113.0/24 metric 999

I removed the executable bit (sudo chmod -x) and logged out and rebooted. My network was auto-started as it should!! I don't know why the release-upgrade put it there (I assume it did), but it was the culprit in this fiasco.