Thank you for your feedback, mr OPBZ, but this is not the case.
I had to do it 4 times in 2 days (and the server has 12 VM's !!! so the users were not very happy) : I do
>>> shutdown -r now
and both NICS start working perfectly ...
All I need is a command to set the NIC "up" and/or "down", the same way I can do "ifup intfce" and "ifdown intfce" .. but it seems it does not exist.
Let me tell ALL the truth : one I was able to fix the problem,
and it was done by changing one-by-one the VM's from one NIC (the not-working one) to the other (the working one).
Suddenly, both NICs started working, .... so this is the line we are following now ...
Also, we have stopped using Ubuntu server 10.10 with ESX 4.0, and have moved back to 9.04, as indicated in "guest OS compatibility guide" :
>>> http://www.vmware.com/resources/compatibility/search.php?deviceCategory=software
Thanks for your time ! Sebastian.