ec2-upload-bundle: Signature version 4 authentication failed, trying different signature version, which fails too

# ec2-upload-bundle --access-key AKxxxx --secret-key xxxx --bucket something-i-own --manifest image1.img.manifest.xml
Digest::Digest is deprecated; use Digest
Digest::Digest is deprecated; use Digest
Digest::Digest is deprecated; use Digest
Digest::Digest is deprecated; use Digest
Digest::Digest is deprecated; use Digest
Signature version 4 authentication failed, trying different signature version
ERROR: Error talking to S3: Server.NotImplemented(501): A header you provided implies functionality that is not implemented

This is: ec2-ami-tools 1.5.7 on Arch Linux latest, running on an EC2 instance.

I understand the Digest related messages are about an obsolete Ruby API that however still works. The NotImplemented is more concerning ...

How can I upload my bundle?