Solution
Capture the Wireshark log and see if you find error 0x8008.
It is a low-level application-layer error.
The error is indicating that the watchdog was set to zero by the master, while the slave expects it to be a different value. Solution is to add this line in the XML:
.../Eeprom>
<ESC>
<Reg0410>#x03E8</Reg0410>
</ESC>
The Register value 0410 indicates the watchdog timer and x03E8 simply a high value given to it instead of the 0 that NI EtherCAT master set.