If an attempt was made to create backup volumes to an external NFS storage that was not available, the creation process would not terminate. This resulted in Block Storage backups getting stuck in a "creating" state. This has been fixed by setting Cinder to detect these cases and create an error.
As a result, backups for Block Storage volumes on NFS storage fail, rather than hanging in a "creating" state. Note that backups are not supported for NFS-backed Cinder volumes, so they will never succeed.