Linux

Run dmesg and see if you find anything suspicious of the cause. If you find something like "blah blah... Ethernet... Blah blah... Key was rejected by service" or similar, it's due to secureboot.

If this is actually related to secureboot your drivers are most likely not in tree and installed via dkms, so they need to be signed or secureboot won't allow them. You can setup a machine-owner-key to sign them yourself, and you can setup dkms to automatically sign them using that key. The instructions are on dkms' readme. After setting up you need to run dkms autoinstall or manually reinstall the drivers to trigger the automatic signing.

Edit: I just noticed you said you deactivated secure boot... I have no clue. But for future reference, you can sign your modules to work with secure boot, it's not a bad idea.

Check in your BIOS if on-board Ethernet is disabled. I think on MSI motherboards it's under "Integrated Peripherals". If not this, check PCI settings in the BIOS.

Interesting typo. Puts your default language to german. :)

But to answer your question: There are two not uncommon things I‘d like you to rule out.

1. defective ethernet port, just now gave up
2. some kind of „wrong port“ situation where one swears they used this one but it actually was another port they used earlier (happened to me once).

If both are ruled out, check if the network adapter is showing „up“ or working condition even without the cable.

I‘ve had a situation where my network adapter was „down“ because I installed it later and the installation (ubuntu in my case) didn’t know it and when I rebooted it defaultet to eth2 instead of eth0 or something.

Good luck.

Try booting into a live image of a debian distro (pick any one you want that can boot into a live image), then try the Ethernet port. Is it working? If not, either your bios shut it off or the port is broken. Several years ago, I had a broken wifi adaptor that happened to break at the exact same time as (or super close thereto) me installing GIMP. No way GIMP caused it. If the port does work on the live image, then it's likely a driver issue with your install. Try it and let us know the results.

In your motherboard settings, are you in UEFI boot mode? Try changing to BIOS/Legacy. Driver firmware can act differently between the two.

Thanks everyone. I tried everything but only with very limited success. I have a very unstable connection that loads very very slow and is basically unusable. I will check tomorrow again if there's damage and if WiFi is affected too. It's already night here so time for a little sleep.