New Backup Format is Not Supported (code 1515)

Situation

A backup plan terminates with the following error: New backup format is not supported.

Cause

The occurrence of this error means that the backup storage selected for the backup plan, does not support the new backup format.

The new backup format does not support the following storage destinations:

Amazon Glacier (not S3 Glacier storage class, but Glacier account)
OpenStack.

Solution

Here there is not much options: the only solution is to change the backup account. To do this, proceed as follows:

Edit the backup plan.
On the Select Backup Storage step, select an other backup storage. The following storage providers are supported:

Amazon S3
Microsoft Azure
Backblaze B2
Wasabi
Google Cloud
S3 compatible.

In case you do not have any of these storage account, click Add new Storage Account.
Follow the backup plan steps to save the configuration.