Baremetal Ethernet KSZ9031RN TX not working on IMX6

Using the SDK for IMX6 with KSZ9031RN chip, and trying to run the Ethernet Loopback Test supported by the driver, I have received this message:

"ENET tx fail"

After receiving this message:

"ENET 7: [ FULL_DUPLEX ] [ connected ] [ 100M bps ]"

Anybody knows where the problem is?

Noting that after debugging I found that the EIR register does not change its value in the transmission loop:

for (i = 0; i < 100; i++) {
   enet_events = imx_enet_poll(dev0); 
   printf("Check: enet_events = %08x\n", enet_events);

   //EIR is the enet_events value, which gives 0 during each loop
   if (ENET_EVENT_TX & enet_events) {
      printf("enet_events = %08x\n", enet_events);
      break;
   }

   hal_delay_us(100);
}

Anyone knows why the TX fails to transmit packets? Or what could be the causes of the problem?

Thank you!