I wrote:

> I will probably try a different hub and different cables but I don't think
> one of those is the cause for my problems.

Instead of cables and hub I changed the network interface card once again.

This time it's a PCI card, the 3Com 3c905C-TX and though it is detected,
it doesn't work:

  ex0 at pci0 dev 14 function 0: 3Com 3c905C-TX 10/100 Ethernet with mngmt
  (rev. 0x74)
  pci_intr_map: no mapping for pin A
  ex0: couldn't map interrupt

I don't know about the PCI-bus at this technical level. What is the
meaning of 'pin A' and what can be done about this problem?

ciao
     Klaus