Hello Rafael,
In order for you to get the best support regarding the behavior you are experiencing, I highly recommend contacting the ARM support team directly, because you are using the CMSIS libraries for ethernet.
Also, I would recommend checking the ethernet examples provided in the LPCOpen, if you would like extra references. However, these examples don't use the CMSIS libraries.
If you would like more information do let me know.
Have a great day!
Sabina