Hi guys,
I really appreciate your suggestions!
I've already checked the BIOS settings and the onboard lan is enabled. In fact, as part of my testing, I've resetted BIOS to default values, without any success.
The only new information came from a utility called Broadcom Networking Boot Utility, which appears after BIOS boot. I entered this utility and there is a command (CTRL-F6) which enable LAN blicking. When activated, I noticed that only the second lan port (bge1) is blinking (no change whether cable is plugged in the first or second lan port).
I must admit this issue _really_ looks as a hardware problem and, as BSDFan666 suggested, I'm on the verge of disabling onboard lan and buying a PCI dual port lan card for server on Ebay.
The only testing left is a careful inspection of the motherboard for short circuit... No real hope there of finding a satisfactory solution!
Thanks again for your time and suggestions..