It seems the app tries to use one bucket name for all instances ever made, which means that it fails to make a bucket at all when trying to backup because the bucket name is already in use.
lazywarden | 2025-06-02 11:00:13,094 - ERROR - Error uploading file to Backblaze B2: Bucket name is already in use: Bitwarden-Drive-Backup
A simple fix would be for the user to be able to specify the bucket name in the secrets just like the other environment variables. Something like B2_BUCKET_NAME.