When does this task start? Does the application make sure that the FreeRTOS+TCP APIs are called once TCP/IP stack is ready?
In the network interface file you are using, how is the configMAC_INTERRUPT_PRIORITY used? Is it passed as an argument to NVIC_SetPriority or directly written to any registers?
Also, I believe you have verified configPRIO_BITS is 3 for your platform (maybe via __NVIC_PRIO_BITS).