When upgrading the pre-installed Recovery Manager for AD agent on the domain controller from version 7.5.2 (and possibly other versions) to 7.6.2, the upgrade completes successfully but any backups fail with "Failed to connect to backup agent: The agent is not installed". When checking to see if the service is running, the 'Quest Backup Agent' is not listed. The executable for the service, %windir%\QuestRecoveryManager\eragent.exe exists and is the correct version (126.96.36.1993)
Re-run the .msi and choose the repair option. This should create, configure and start the 'Quest Backup Agent' service.
Uninstall and re-install the pre-installed agent.
NOTE: Upgrading the pre-installed agent from version 7.5.2 to 7.6.2 will require a reboot unless you stop the 'Quest Backup Agent' before starting the install. The repair and/or re-install process may require a reboot as well.
This issue with the upgrade will be resolved in version 7.6.3. The release date has yet to be determined.