Very suspicious indeed, yes, 1 packet every 5.
I've tried _many_ things, by stopping almost _all_ services, changing the IP address, making sure DOS protection was not enabled, moving the Microserver to another switch, changing the virtual NIC within ESXi as well as the physical NIC (I've got an Intel dual Gig also in the Microserver) and ... no change.
I finaly gave up, deleted the Xpenology VM, created it again (with 4 VMDK for the time being and not yet 5 but that should not change anything), and now it rocks ! (I'm afraid that I'll never understand what happened...)
Thanks for the tips anyway.