In the System tab -> System Log page you see to conflicting messages - see attached. There will be a message saying that the backup failed and the next message will be that the backup succeeded.
The cause appears to be a logic error in the checking of the backup status. The failed message is genuine, the succeeded message is incorrect.
Workaround / Solution:
The SUCCEEDED message is wrong - it should not appear, however the FAILED message is genuine and should be investigated. You will see the reason fro the failure in the log and you should take corrective action based on the reason given to ensure that backups are successful in future.