I suppose this doesn't have much to do with ATK in itself, but let's hope things work better if you add nano6 to the /etc/hosts file:
1.2.3.4 nano6
As mentioned, we upgraded MPICH2 in ATK between 12.8 and 13.8, which apparently triggers this behavior. Thanks for reporting it, it was quite unexpected. It's not an error, but good to know in case someone else has similar issues.