update: checked and confirmed: aws_calling_format: SUBDOMAIN in s3config.yml yields a 403 (forbidden, Error: SignatureDoesNotMatch)
I have this problem also.
This works:
AWS_CALLING_FORMAT=SUBDOMAIN ./s3cmd.rb createbucket mybucket EU
but only if I remove
AWS_CALLING_FORMAT: SUBDOMAIN
from s3config.yml. Otherwise it fails with 403 Forbidden (SignatureDoesNotMatch)