A DCS 6.8.x Linux Agent was installed with fallback definitions only and has never received any newer updates.
When such Agent is upgraded to version 6.9.1, AutoProtect shows Disabled status in UMC Console.
Agent logs indicate that Scan Engine could not be initialized due to missing antivirus definitions.
DCS 6.8.x Linux Agent upgraded to version 6.9.1.
The AV scan engine has been updated in version 6.9 and requires an updated set of fallback definitions.
During upgrade, use the -InstallFallback=<path_to_fallback.tar.gz> parameter to install the 6.9.1 fallback definitions.
The file can be obtained from THIS KB ARTICLE
Once the Agent is upgraded successfully, AutoProtect will display Enabled status in the UMC Console.