You’re right!
Backblaze does not support x-amz-checksum-crc32, and Discourse’s newer AWS SDK version may have enabled this by default.
so i went into the app:
./launcher enter app
and uninstalled the current version of AWS SDK:
gem uninstall aws-sdk-s3 aws-sdk-core aws-sdk-kms
and installed the one that the backblaze guy said would work:
gem install aws-sdk-core -v “~> 3.215.1”
gem install aws-sdk-kms -v “~> 1.96.0”
gem install aws-sdk-s3 -v “~> 1.177.0”
then I rebuilt the app and it worked!