Getting this error most possible because SQL Server Backup Manager has not to refresh license information from Backup Agent yet.
If your Backup Agent has been registered before the upgrade, you don't have to re-enter your license key once again after upgrade to the latest version even if the backup manager shows it is in the trial period. You just re-connect the backup agent in the "SQL Instances " configuration window to refresh the license information from the backup agent. See the following.
If above step still dose not work , please use following steps to resolve this issue :
1 . open "SQL Instances " window , and click the "Manage backup agents".
2 . write download the alias , and address information of the backup agent that has license issue . we will delete then re-create it.
3 . delete the backup agent that has license issue to delete the conflict license information.
4 . add this backup agent information once again .
5 . re-enter the license key for this backup agent ,in "Help->Enter registration code" menu FOR THIS backup agent.