The error "Class not Registered" is returned when attempting to synchronize a mailbox in Enterprise Vault (EV) for Microsoft Exchange.
Source: Enterprise Vault
Category: Task Controller Service
Enterprise Vault Task Controller Service
Unable to synchronize with an Enterprise Vault Service.
The service is not present in the target computer's NT Services configuration. The Enterprise Vault site may not be fully configured, or the service may have been deleted from the NT Services configuration.
Service database name: Enterprise Vault Directory Service Configuration of the Enterprise Vault site should be completed prior to starting this Enterprise Vault service.
One or more Enterprise Vault DLL files or services have been unregistered.