When running Veeam Agent for Microsoft Windows (VAW) 2.1 backups to a Veeam Backup and Replication (VBR) 9.5 repository, you can encounter the following behavior after upgrading VBR 9.5 from U2 to U3.
All VAW machines are moved to the Unmanaged group. When trying to move these machine to another Protection Group (right-click > Move to) you receive the following message:
This is normal behavior. After upgrade, VAW is not aware about new version of VBR. The next time VAW connects to the VBR server, the agent status will be updated. It can happen on the next backup job run or while refreshing the license information.
If waiting for the status update to happen automatically is not an option for you, open VAW control panel, click Configure backup and go through all the steps in order to refresh the status of the VBR server connection. After that you will be able to move your computer to the protection group.