Ethernet not working after installing Security Update 2020-002 10.14.6
As follow up to this very frustrating problem ...


On further analysis, the problem is that ONE of the USB-C ports stopped working (for both ethernet adaptors, and also for standalone USB to USB-C adaptors). The other USB-C port remained working fine.
After shifting the power input to the 'dead' port, that appears to have reset it, and it is now working again. I am suprised that this helped when re-starting the computer etc made no difference.