When creating API make sure you check _Allow List All Bucket Names_ if you allow access to a single bucket only.
If you use a _File name prefix_ when making the application key, do not include a leading slash. Make sure to include this prefix in the path (e.g. `path: 'backblaze_bucketID:my/path'`).
Also make sure that the _File name prefix_ (if used) does not includes a leading slash.