Hello LukasB, thanks for joining the Motherboards community.
Have you tried the procedure provided on the other thread that you mentioned? https://downloadcenter.intel.com/download/22026/NVM-Update-Utility-for-Intel-82579V-Gigabit-Ethernet-PHY-Network-Connection
It looks like this issue is similar to the one you are experiencing.
Something very important that I found out is that the DX79SI board comes with Intel 82579L and Intel 82574L Gigabit Ethernet Controller; therefore you might be having the same issues described in the URL.
I would recommend you uninstalling all drivers and install them again.
If you continue having issue you can always contact our Warranty Department so they can help you replacing the motherboard, http://www.intel.com/p/en_US/support/contactsupport or submit a web ticket at the following URL
Thank you for your care. I tried that patch from 82579V before, and also tied it again now with same result:
No update required.
If I good understand, this patch is for 82579V network cards, that accidentally change driver/identification as 82579LM. But my card should be real 82579LM. That's why patch not works in my not very deep knowledge.
I also tried uninstall drivers and install again. Same result
I will try submit the request but support ends in December 2014