macOS - How do I remove the restart-time update installer?
Re-installing as suggested above will fix the problem - temporarily! The next day you will end up in exactly the same situation, unless you stop all automatic updates from taking place.

The cause of the problem is a BUG in the 2019-004 10.13.6 High Sierra update, affecting my Mac Mini Server mid-2011, and obviously more mid-2011 machines.

I have forwarded the info to Apple, but don't hold your breath wile waiting for a correction. Mid-2011 machines are close to extinct and all updates for those will soon, if not already (?) stop from being developed.