Just ran into this with 1.2 RC9. eth0 and eth2 swapped devices, presumably due to differences in the timing of device init in the new kernel version, meaning DHCPd ran on the WAN interface, which the upstream won’t appreciate. Having proper static device naming would have prevented this.

@gadams I could have sworn I'd put $ifname in there, but the git history doesn't lie; apologies for missing that and thanks for catching.

Resolved by waiting for IPv6 DAD to complete in scripts/vyatta-dhcpv6-client.pl. The 'configure failed!' message no longer appears, dhclient starts and the configuration nodes are not deleted.
Typically it only has to wait 1 second for DAD to complete, so on some systems/reboots you would never have seen this issue.

The relevant Debian bug is https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=834928
From this report, the issue is in ifupdown and is supposedly fixed in 0.8.11. Unfortunately, this version is not available in Jessie without resorting to backports.

Some further investigation: