This one had me stumped! Thankfully this Debian bug report about netbase setting net.ipv6.bindv6only=1 in /etc/sysctl.d/bindv6only.conf cleared up the problem for me.

The symptoms included the error:

java.net.socketexception / network is unreachable

whenever I tried to refresh the mirror list, update a plugin, or install a plugin. The problem was fixed for me after I ran the command suggested on the Debian bug page:

sudo sed -i 's/net.ipv6.bindv6only\ =\ 1/net.ipv6.bindv6only\ =\ 0/' \
/etc/sysctl.d/bindv6only.conf && sudo invoke-rc.d procps restart

¥