Scheduled backups are failing to occur.
Each customer is assigned a given amount of storage space to store backups. If the amount of space required to store your backups has reached that limit, your scheduled backups will fail until you either have more storage space allocated to your account or reclaim some of your existing space by deleting some of your previous backups.
You can reclaim space by unregistering a server, by deleting a backup schedule, or by changing the retention range of your backups. Backups that are flagged by running one of these operations will be deleted when the online backup service runs its clean up processes. Clean up happens once a day, so it may take up to 24 hours for space reclaimed this way to become available.
Alternatively, you may contact Microsoft Support to request additional storage space for your account.
Windows Azure Online Backup Errors and Events List