Ended up just uninstalling and reinstalling, although that would hardly be a viable solution in environments where SupportAssist Enterprise was monitoring several devices.
I have a very simple setup here, and somehow SupportAssist Enterprise can't handle it.  It's a PowerEdge T330 running Windows Server 2016 Essentials -- so this is an AD domain controller if that matters.  I've got OMSA 9.2 and SupportAssist Enterprise 2.0 installed on this server, and all I want to do is add this server to its own SupportAssist Enterprise instance.  I created a credential profile for "Servers" using my own domain admin credentials.  I'm able to use the "Add Device" function to add the server to the Devices list successfully, but its Status column shows as "Failed", with a note that inventory validation hasn't been performed.  When I select it and click "Validate Inventory", it runs for a while and eventually comes back as failed, with Error Code SA-0015.  The documentation says to verify that this is a supported device and that you're using admin credentials.  I've verified both of those things.  I've tried deleting and re-adding the device to SupportAssist Enterprise multiple times, deleting and recreating my credentials, and stopping and restarting the SupportAssist Enterprise services.All I want is to use SupportAssist Enterprise on this server so that it generates automatic cases in case of hardware problems.  Why is it so difficult to get SupportAssist Enterprise to work properly on the server that it's already installed on?  This all worked fine prior to the 2.0 release, by the way. I noticed this was broken after what must have been an auto-update to 2.0, since I never installed or approved that update, and I haven't been able to get it working again.  This is meant to be the simpler version of OpenManage Essentials that's intended to be usable by small businesses that might not have dedicated IT personnel?
Error SA-0015 trying to add the local server