Hello Patrick,
ok, i just tried to ping the domain controller, not the gateway. The ping gave no result, like a not working nic. The Nic change from "no internet" to a red sign with "no network". after VM reboot. VM shutdown, move to the other Host, reboot, VM is reachable. During all this testing the other VM's on the "new" ESXi 5.5 U3 Host worked without problems!. So really strange.
All The Problematik VM's have VMXNET3 Network Cards. Because the one is the File Server and the other the ERP System Server and DB server (license is pined on Network-MAC of the 32Bit Server), i was not willing to do some more "trial and eror" to get more Problems.
What do you mean with "Do you "see" the mac-address of the VM on the switchports of the source or destination ESXi host?" Maybe you ask me in our native language at vmware-forum.de? :-)