@julien66 I have the same use case as you and this was a head scratcher.
I think there are 2 issues here:
(1) On backblaze b2 app key creation, I have contacted their help desk and they have responded that the appkey must include all buckets for the s3 endpoint to work
(2) On backwpup, since the appkey would not be valid, this then throws the error as you described, and there is no way to delete the keys as it is cached and stored in the DB I think.
To workaround this issue (and I have tested that it works) – create new appkey on b2 with all buckets permission, and recreate new job on backwpup and it is critical that we fill up the values in this order: s3 server url, access key, secret key.
side note – there may be a bug somewhere on the plugin on the s3 menu not loading due to error by an invalid key