iMac(m1) ethernet not working after upgrade to 14.0 (23A344)

I'm replying to this hopefully help anyone who may have the same problem as me. Just installed at home a new FTTH fiber line . I immediately noticed modest performance, but the thing that leaves me most disconcerted is that the value of Ethernet is significantly lower than that of Wi-Fi ( 94 mbps / 240 mbps), something I can't explain. I have a latest generation iMac 24 with the Ethernet port in the power supply and Mac OS Sonoma with clean installation. I tried deleting the ethernet port and adding it in System Preferences but nothing changes.

I spoke to Apple support 2 times. The first time they suggested I change the cable. Cable replaced but no change. I spoke to Apple support the second time and we tested connecting a Macbook (which returned 940 mbps with speedtest). We came to the conclusion that it could be a hardware problem with the Mac or the power supply (with Ethernet).

I had made an appointment for today at an Apple service center to have the Power Supply or iMac 24 checked and I had also made a backup.

So everything led to the 2 hardware elements...then I made one last attempt: the classic test of add a new user and log with this one and there I see that the connection reaches 940 mbps.

Having then confirmed that the general preferences are fine, I go to delete all the network preferences in "Preferences" of my account and delete the Ethernet "service".

I restart and once the Ethernet service has been restored everything is fine now the Mac sees me at 940 mbps, what I don't understand is how this could happen given that I did a very clean installation

of Sonoma without import any kind of preferences. Thanks for reading

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