Sometimes when you remove an infrastructure object (vCenter or Hyper-V server) from the Veeam ONE Monitor client, the removal process may take longer than expected.
Once you click the Remove server button from the infrastructure tree view, it could take some time to remove the data from the database properly.
The required time depends on the SQL Server performance and the amount of data Veeam ONE database contains.
In order to speed up the removal process, stop the Veeam ONE Reporter server service until you can see that the object in question is no longer shown in the infrastructure tree. Once the object is successfully deleted, start the Veeam ONE Reporter server service.