After updating to Platform 4.18.2008.4, alongside with KB 4566782, the error messages stopped, the Windows Defender Antivirus Network Inspection Service is up and running on my system. Core isolation is on.
I don't know where to find the update (tried searching microsoft web site). I don't know if these are applicable to Windows 1903. Should't these be updated automatically if they apply to my system? The notes on KB4566782 seem to imply it was included in the August 11 update, which I do not have yet. Is the Antimalware Client Version 4.18.2008.4 in that update? If not, does it apply to my system, and if so, where exactly do I get it, and why was it not part of the "normal" update process?
Go into the Windows Search bar / Cortana and type Update, or however you navigate to Windows Update, and force a manual check for updates. It may take a little bit for your machine to be ready but that's how I did it. When I booted this morning, I had no updates when I opened it initially. Forced a check, and it found 3, a .Net update, the monthly rollup, and a defender antivirus definition file.