In Japanese Cygwin, the default group is "なし", which means "none" in Japanese, and this includes non-ASCII characters.
Because of this, s3cmd sync fails by this error.
ERROR: S3 error: 403 (SignatureDoesNotMatch): The request signature we calculated does not match the signature you provided. Check your key and signing method.
If I change my group name to English "none", this does not happen.
Also if I change preserve_attrs to False, this error does not happen too.
I think if I use non-ASCII characters for my user name, this happen too.