Cluster launcher stuck on "Checking AWS environment settings"?

When I change:

provider:
    type: aws
    region: us-west-2
    availability_zone: "us-west-2a,us-west-2b,us-west-2c,us-west-2d"

to

provider:
    type: aws
    region: ap-northeast
    availability_zone: "ap-northeast-1a"

the cluster launcher gets stuck on “Checking AWS environment settings”.