I had the same problem that I tracked down by adding --verbose to the ifup commands.
This led to /etc/network/if-up.d/mountnfs Adding -x to the shell options showed that the hanging command was "mount -a -t$NETFS" which was executed as "mount -a -tcifs" It turns out I had a filesystem mount in /etc/fstab of type cifs that required a password and that lacked the "noauto" keyword.
