It looks like I’ve got an old version of aws-sdk-s3.
$ bundle info aws-sdk-s3
* aws-sdk-s3 (1.177.0)
Summary: AWS SDK for Ruby - Amazon S3
Homepage: https://github.com/aws/aws-sdk-ruby
Source Code: https://github.com/aws/aws-sdk-ruby/tree/version-3/gems/aws-sdk-s3
Changelog: https://github.com/aws/aws-sdk-ruby/tree/version-3/gems/aws-sdk-s3/CHANGELOG.md
Path: /var/www/discourse/vendor/bundle/ruby/3.4.0/gems/aws-sdk-s3-1.177.0
Looking into this reminded me that I used this workaround to get around an issue with AWS SDK not working with Backblaze. Catching up on the topic, it looks like Backblaze updated their API, which makes the workaround unnecessary.
After I removed the workaround, the backup succeeded! Thanks for the help. 