The Symantec Management Agent service was not started. Tried to start it manually, but got "Error 2: The system cannot find the file specified." Rebooted server, no change. Confirmed that Program Files were present, and also tried to start from the command line using AexAgentUtil.exe /start, but that did nothing.
AeXNSAgent.exe was missing from the Altiris Agent folder
Copied AeXNSAgent.exe from another managed computer at the same agent version.