When running lwIP on FreeRTOS, the port layer is not setting the interrupt priorities for the ENET interrupts as needed.
The port layer contains this code:
install interrupt handlers...
/* Set priority for ISRs */
It sets the interrupt priorities correctly if you are using MQX, but misses it when using FreeRTOS.
Also, if you are following Erich Stygers tutorial, you need to make sure the "Make SysCall Interrupt Priority" setting in FreeRTOS component is configured to have a higher priority than the ENET interrupts.