Web Demo Stop working with lwIP

Hi, I just downloaded the latest FreeRTOS 5.1.0 release and try to run the web demo with lwIP using ARM STR91x.  Everything worked well at the beginning and I was able to see the web page refreshed itself.  After several hours of running (5-6 hours), the web page could not refresh anymore.  It seems that the lwIP stack has stopped responding to any SYN request from the browser.  I am not sure whether this a RTOS problem or a lwIP problem.  I didn’t change any code except commenting out all the tasks except the "BasicWeb" task in main.c.