marcofognog/ungulate — comment on commit 45c6ff722817d8f3b645148b5a3d5578677f8b33
File:  line 0
By camelpunch:

Because you were using a bucket that didn't require redirection, so not setting s3_region would default to us-east-1. I was testing with an eu-west-1 bucket, which required redirection, and hence triggered the failure.