I have contacted Nexcom support but they have not come up with any good answer so far.
If I disable symmetrical multiprocessing the problem disappears.
It seems like that the driver (PRO1000 on WEC7) in the package “Intel® Ethernet Drivers for Microsoft* Embedded Operating Systems version 12" does not work correctly when symmetrical multiprocessing is enabled and there are several Ethernet adapters.
I have tracked down the text printed to e1i61ce7.dll which comes form the package above.
Also if I query the adapter with IOCTL_NDISUIO_NIC_STATISTICS only the first adapter reports any packets and bytes but they all shows correct for mediastatus.