I've played a little with KSDK MQX httpsrv_demo application focused to websocket part as it is my primary interest and found the application starts crashing randomly with corrupted memory. So I've tracked it down to the function httpsrv_plugin_task() in tcpip/rtcs/source/apps/httpsrv_task.c where unitialized pointer pl_context is accessed (tested to NULL). The bug is simple fixed by initializing the pointer to NULL.
Hopefully this helps somebody
PS: The websocket implementation is quite good as it passed almost all tests in AutobahnTestSuite.