This error indicates that a packet was received for a VLAN that is not defined in iPXE.  For example, if you have created only VLAN 5 on interface ''net0'' using

    vcreate --tag 5 net0

and then a packet is received on interface ''net0'' with a VLAN tag of 6, then you will see this error show up as a receive error ("RXE") for interface ''net0''.