However, any further suggestions would be appreciated if for no other reason than to solve the puzzle.

WiFi works fine and is almost as fast as ethernet, so I can live with this minor issue. Since re-installing MacOs didn't help, it isn't a piece of Apple software causing the problem. Having tried unloading or disabling all that I am aware of, though, I guess I will just have to wait for the mystery software to be updated some time in the future. It must be some piece of software that I am running. Unfortunately it still did not solve the original USB ethernet problem. Ran a diagnostic and no trouble with the ports. That solved the diagnostics screen problem.

I then updated the firmware using Congifurator 2 on my iMac and putting the MacBook Air into DFU mode. Did not solve either the diagnostics screen problem or the USB ethernet problem. They suggested re-installing MacOs which I did. Any suggested fixes would be greatly appreciated. I just can't figure out what is gumming up the works. I do not believe a re-install of MacOS would help since my existing version (11.3.1) does work fine in safe mode. I can live with the situation, but it is inelegant. There was a time when ethernet through USB worked intermittently but for the last few weeks (with no new software installed), it hasn't worked at all.

Assigning a fixed IP address rather than DHCP (something suggested in posts late last year) didn't help either. I have tried disabling or uninstalling everything that starts up on log-in including prime suspects lime me Intego firewall, all to no avail. When I go into safe mode internet access is perfect with a USB to ethernet adapter. It just is kind of clunky for travel, etc. WiFi works fine for everything, and when I default back to an old Thunderbolt 2 - ethernet dongle, connected to another thunderbolt 2 - thunderbolt 1 dongle that works fine too. However, all connections to the internet time out. I've tried multiple brands with different chip manufacturers and the same thing happens: network settings recognizes the USB ethernet adapter, assigns it an IP address and it works fine for all LAN activities. Everything works flawlessly except that no USB ethernet connection will work properly. I have read posts about this several months ago.

Ethernet adapter apple macbook air