Recently I began to observe the following scenario:
After the Kubuntu has booted the icon for wired ethernet is red.
And there is not "Connect" menu for the corresponding connection in network manager.
However after unplugging the cable and plugging it again the icon becomes white and ethernet starts to function.
The problem doesn't happen after each PC start and I'm sure that the cable is ok. There must be some unhappy timing at OS level that causes it. And I also think that it all started after I renamed the connection from "Wired connection 1" which was the default after Kubuntu installation to just "Wired connection".
I can try to use another ethernet cable in the meanwhile but I'm 99% sure that the problem is caused by this "unhappy timing" in OS. I tried to bend the cable when it was plugged but the connection functioned.
I tried to restart the services networking and network-manager but it didn't help
P.S. I used another cable but it didn't resolve the problem