The solution is to try and reset as many network related system preferences as possible. To be more specific:

1. Unplug the Thunderbolt Ethernet adapter.
2. Create and Apply a new default network location (system preferences).
3. Remove all older pre-existing network locations.
4. Remove the following preference files (global and user specific found in /Library/Preferences /Library/Preferences/SystemConfiguration and ~/Library/Preferences):

com.apple.network*.plist NetworkInterfaces.plist

5. Restart your Mac.
6. Add a new Location inside the network system preferences (AGAIN!) and remove the one added previously in step (2).
7. Connect your Thunderbolt Ethernet adapter as well as the network cable.
8. Wait a few minutes (close the system preferences).

Step (6) and (7) may be the other way around. Here is what I observed while trying to solve my issue:

I always used to have two Thunderbolt related entries in my network settings:

1. Thunderbolt Ethernet
2. Thunderbolt Bridge

I believe the bridge may be added by default and the ethernet version may be added on demand, when you plug in an ethernet adapter.

While my connection was "broken", however, this Thunderbolt Ethernet service / interface never appeared or worked again, so possibly some of the settings files related to it were broken.

After removing all old references (from the old locations) to it as well as the listed preference files, I now get the following three default services listed, when adding a new location into the network preferences:

1. Thunderbolt Ethernet
2. Wi-Fi
3. Thunderbolt Bridge

If you don't get these three and you are using a Thunderbolt Ethernet adapter, you may have the same problem as I had and should follow the advice above.

The solution is to try and reset as many network related system preferences as possible. To be more specific:

1. Unplug the Thunderbolt Ethernet adapter.
2. Create and Apply a new default network location (system preferences).
3. Remove all older pre-existing network locations.
4. Remove the following preference files (global and user specific found in /Library/Preferences /Library/Preferences/SystemConfiguration and ~/Library/Preferences):

com.apple.network*.plist NetworkInterfaces.plist

5. Restart your Mac.
6. Connect your Thunderbolt Ethernet adapter as well as the network cable.
7. Add a new Location inside the network system preferences (AGAIN!) and remove the one added previously in step (2).
8. Wait a few minutes (close the system preferences).

Here is what I observed while trying to solve my issue:

I always used to have two Thunderbolt related entries in my network settings:

1. Thunderbolt Ethernet
2. Thunderbolt Bridge

I believe the bridge may be added by default and the ethernet version may be added on demand, when you plug in an ethernet adapter.

While my connection was "broken", however, this Thunderbolt Ethernet service / interface never appeared or worked again, so possibly some of the settings files related to it were broken.

After removing all old references (from the old locations) to it as well as the listed preference files, I now get the following three default services listed, when adding a new location into the network preferences:

1. Thunderbolt Ethernet
2. Wi-Fi
3. Thunderbolt Bridge

If you don't get these three and you are using a Thunderbolt Ethernet adapter, you may have the same problem as I had and should follow the advice above.