I resolved the problem with MSP support.
Follow this steps:
1) Close the Backup Agent and stop MSP services
2) Rename your C:\ProgramData\Online Backup\Data to C:\ProgramData\Online Backup\Dataold
3) Restart MSP services and open the Backup Agent.
4) Start a Consistency Check for your storage account
5) Once done, go to Backup Storage tab > find the plan than generated the error > click the generation and check whether you see "Additional metadata retrieval is required" message
6) Click Run now button
7) Once the retrieval is done, try re-running the backup once again

Hi guys.
I've got the same issue.
Any news?