Seems it was unrelated to vSphere/Ubuntu..
After a little investigation I found:
We had a Linn player connected, but off.. which caused this weird behaviour..
I pinged the virtual ubuntu server, got a ping reply from the server and then (with same sequence #) from a 0.0.0.0 IP with a Linn MAC-address..
same with a HTTP request.. reply from the server, then garbage from the Linn..
I asked the AV guys here and we unplugged the power from the Linn devices (one by one) and found the culprit.. dunno why that player interfered in such way - never seen anything like that.
Anyways.. Cheers,
--
Thomas