Thanks for your reply! 
So they are defined as global variables,
StackType_t xStackManager[STACK_SIZE];
StackType_t xStackControl[STACK_SIZE];
with
#define STACK_SIZE 2000
I have to admit that it is not clear how to define the stack size I should look at it thoroughly. I also tried 500 and it seemed to work as well 
I read that there is a function to check the available stack size but I m not sure on how to use it. Definitively something to master I guess.
I thought that configASSERTwas defined but now I doubt about it. I will have a look and tell u asap.
I ll give a try to configUSE_PORT_OPTIMISED_TASK_SELECTION then.