EthernetIP extended error codification is a standard, not Fanuc-specific. Here is the error description from an Hilsher documentation :
0x126 | This extended status code is returned when the target device determines that the data
segment provided in the Connection_Path parameter did not contain an acceptable number
of 16-bit words for the configuration application path requested.
I had similar experience with a Balluff device that needed extra configuration information to work. It's a list of bytes you should find either in the device's eds file, either in its documentation (you may have to mix both...).
Fanuc EthernetIP menus can't handle this extra configuration bytes, so once you get the byte list, change the following system variables :
$EIP_SC[x].$CONFIG_SZ = number of bytes in your list
$EIP_SC[x].$CONFIG = byte values from the list
where x is your device number from the Fanuc EthernetIP menu.