- Another system with similar symptomatic was processed in the same way (Agent uninstall - McAfee products removal tool - Reboot - Agent deploy via ePO - VSE deploy via ePO - Reboot - DAT update), and result was the same. System also updates regularly.
Seems the issue was caused not by agent 5.5 only, but to remove that issue a single component change is enough. Great thanks to Everyone involved!
Just in the past week I have had the updater problem return on one of the Exchange servers that I had previously fixed. DAT updates stopped once KB4493446 was installed. I've asked our Exchange admins if we can remove that KB in order to see if that fixes the problem.