I turned off the shields (to run a Windows update) by right clicking on the Avast icon in the system tray and selecting “disable until computer is restarted”. The computer restarted but the shields didn’t, and had to be started manually.
On 2 computers both running Windows 10 x64 version 1803, both Avast Free 18.5.2342.
This used to work but the latest version seems to have broken something.